Learn about "European Common Framework for Languages" levels
You'll have to get a quick, broad "intuitive" feel for what to teach at each level (again: "broadly" speaking i.e. "A," "B," and "C" or elementary, intermediate and advanced). I don't think most courses will ever go very deeply into what students have to know at each level or the whole examination/evaluation process because there are loads of other things they'll prioritize and they'll usually assume that your DOS (Director of Studies) at whatever school you end up working at will handle all that anyway; but it's very, very important (and the reality may be that you'll find out that your DOS/school doesn't seem to handle placing students in the correct levels anyway). You can get a quick feel by sitting down with a complete series of textbooks (maybe at the school where you end up doing your TEFL course), which are usually titled "beginner" (A1), "elementary" (A2), "pre-intermediate" (B1.1), "intermediate" (B1.2) "upper-intermediate" (B2), "advanced" (C1), and "proficiency" (C2), and going through the contents lists of topics and vocabulary areas covered, as well as the grammar sections. As I pointed out earlier, just about all of the textbooks available nowadays will state somewhere (in the teachers' book, maybe) where they feel they fall on the European Common Framework for Languages.
Learn how to evaluate students
You'll have to know a little about how to evaluate students' levels (see above). Remember that most TEFL courses will be aimed at preparing teachers for teaching in pre-existing language programmes in institutions, whether neighbourhood academies on the one hand or universities on the other. Institutions other than companies/corporations will have a strong pre-existing structure set-up for evaluating students and it'd be a good idea for your TEFL course to cover this type of thing as well, but again they probably won't do so in depth because they'll be thinking of your DOS again, and of the exams that are usually included with the coursebook series. (The CELTA, for instance, does not prepare teachers specifically for in-company classes, which is probably why the audio-lingual method ever got and kept a foothold there in the first place. There is no strong pre-existing set-up for evaluations in in-company classes, such as there is in many other institutions.)
The fact is, however, that in these institutions, you'll be teaching students how to pass exams, so that's what many of them will be most interested in anyway, usually enough of a critical mass of students just want the "certificate" to make it a significant factor in how you'll have to teach your classes. Exams tend to have sections covering "grammar," "vocabulary," "reading," "writing" and "listening." To a lesser extent, because of the logistics involved in evaluating the speaking of large numbers of students, they may also have a "speaking" section.
So, again, you'll have to cover "grammar," "vocabulary," "reading," "writing," "listening," and "speaking" in your English courses because these last 4 skills will usually end up on exams, together with the grammar and vocabulary that's necessary to be able to "write," "listen," "speak" and "read." In general, TEFL courses will cover how to teach all of these points to one degree or another as will every general course book in existence.
Learn what to teach (focus on / aim at) in your classes
The reality of the examination process is what students perceive to be accepted practice and this will have an effect on what they'll expect to learn in your classes. From long experience, they'll expect to see a certain structure on the exam. On the exam, for instance, students will expect to see a larger number of multiple-choice questions and fewer gap-fills. They might, for example, expect to see 25% grammar, 25% vocabulary, 20% reading, 20% listening and 10% writing. If there were a speaking section on the exam, it probably wouldn't be worth much more than 15%. If you were to plan on varying this structure much, it might be a good idea to let students know beforehand.
In fact, students usually like to know how you plan to structure your whole course on the very first day of class. For example, they will want to know what homework they will have to do and how much it will be worth. (Usually, your school will already have a system in place which you will be expected to follow.) Then, they will go through the whole system and ignore all the parts they're not the least bit interested in. You might, for instance, assign the coursebook workbook for "voluntary" homework, but don't be surprised if not a single student does it even if and when it's obligatory. The same goes for writing homework. As a matter of fact, expect to always have your students disect your grading system and do only the parts they want to do.
Basically speaking, expect a significant number of "minimalist" students in all private institutions, even in private universities. These are students who, for example, always ask you what grade they currently have right before the final exam so they can find out the minimum grade they need in order to pass the course. It's amazing how well some of them can nail the minimum grade time after time. Basically, what this means is that you get expert exam takers who do just well enough at taking and passing exams, but who don't really have the corresponding level because they never go the extra mile. For example, you can hardly understand them when they speak, but they're supposed to have an intermediate level on paper.
(By the way, I've many seen students just barely pass my classes one year only to fail miserably the next. Many textbooks in English courses take up where the previous textbook in the series leave off "grammatically speaking", but the language content usually somehow seems much denser in vocabulary, speaking, reading and listening. The students with the minimalist mindsets usually come up against a wall at some point. In other words, it becomes overwhelming.)
Learn about what motivates/moves the students themselves
So, I believe another area you should hit on in a TEFL course is on the right balance between entertainment (play) and teaching/learning (work) because you may come from a background like me where work is held in higher regard than here. There's no question that a few other nationalities (Germans, northern Europeans?) are better at learning languages than Spanish people (I'm sure it's because they work at studying more), so I think it would be a good idea to do your TEFL course in Spain with Spanish people, who in my experience generally detest studying (i.e. memorizing vocabulary and so on) outside of the classroom.
This is further complicated by the fact that English courses have a reputation as being "Marias" in Spain, which in Spanish basically means, "an easy course to do". If you challenge this expectation, expect to get seriously trounced by your students' evaluations, even 3 or 4 failing "Maria students" or "Marias" will be enough to lower your own evaluations enough to get you the silent evals-lynching treatment.
Here's a story about a NYU Professor's Class Cheating. Basically, 20% of the students in his classes were caught cheating on exams. He pursued the matter and as a result took a whipping at evals time. Low evals at NYU receive lower pay. Notice that the professor's first reaction was "never again", which is what I think the great unwritten purpose behind the whole evaluations system. I think the idea is to get professors/teachers to ease up on the students and pass as many as possible because they'd otherwise lose the failures' payments.
Read the abundant comments below the article too, for example:
"Now, the colleges are all in competition and each class is a revenue center and a popularity contest, all about making the kids feel good by giving out good grades for poor performance and little learning, since the student is the "customer." That's why you see college graduates who can't even write complete sentence or think clearly. Society, employers and the students are the losers in the "feel good" system."
bobzillah killah
By the way, spoiled kids like the ones I've mentioned grow up and infect the workplace in mass with their excessive sensitivity to "horrible bosses" (like in the movie) and their inability to be honestly productive continues to go out the window.
For example, I have caught many students cheating here. One thing I will never do again is accept texts printed out from a computer. Many students will use Google translator and not even check their own work (i.e. translate a text they've written in Spanish using online software). This is irritating because every other sentence will have mis-spelled words in Spanish which the translator will, of course, not translate. Also, direct translations usually sound totally absurd. Another thing they will do is to find texts online and just plagiarize them and hand them in "as is" or with minor changes so as to throw you off if you search for bits of the text online. Expect them to copy things word-for-word off of Wikipedia for an oral presentation, for instance. This is irritating. Now, you know that students can't possibly hand in perfectly written homework with vocabulary and grammar that go above and beyond what anyone in their level would normally be able to do. As well as all this, be aware that brothers and sisters (boyfriends and girlfriends?) will sometimes do your students' homework for them and at other times groups of students will do the homework together. This consists of one student doing all the work and then singing off the answers to the others. (You'll sometimes notice that every last one one of them will have the same exact series of letters in an incorrect answer. eg. they'll all have the answers in this order: "B", "D", "C", "F", "A", "E", when the correct order is "A", "B", "C", "D", "E", "F". So much for any expectations that students are getting anything out of your workbook homework! Unlikely if they're cheating.)
How do students like this do in the workplace? Probably like the Marx Brothers. So, why are they so tolerated?
By the way, here's a similar bit of Stateside news in the Huffingtonpost (Nov. 23, 2011): Texas School With 'Exemplary' Status Only Taught 2 Subjects, Faked Other Grades. Here's another one (Nov. 25, 2011) about universities in Spain in Spanish: Cursos originales, extraños o de dudoso contenido en las universidades españolas. (Strange, "original" courses or with doubtful content in Spanish universities. Some of them are pretty friggin' strange.)
I think that it is a sad fact of life that private for-profit institutions may very well have a vested interest in passing students at just about any cost. At a university level, for instance, almost every student will want to attend one of the public universities as they're almost free. Private institutions, in other words, aren't getting the best and brightest anyway. But, the fact is that most highschool-age kids are failing exams such as the CDI (Conocimientos y Destrezas Indispensables - Indispensable Knowledge and Abilities) which tests the minimum knowledge that kids should have to be in school. 70% of them failed this year and I recall similar results on similar exams going way back. As far as I know, it's generally accepted by the locals that students pay to pass in private elementary schools, high schools and universities so I believe the practice is widespread because these students simply cannot pass any other way since they don't even have basic study skills.
So, it's no surprise that private institutions would put so much stock in students' evaluations of teachers, who often prepare their own exams here, by the way, thereby virtually guaranteeing easily passable exams as they are "virtually" held hostage (i.e. low grades = bad teacher; good grades = good teacher). Public schools do no such thing and students/parents must report "bad" teachers to inspectors and principals or harrass them to get anything done about them.
By the way, my own 8 year-old son attends a public school and has to do literally 3-4 hours homework every day. Many of the Spanish parents in the class would like to stop that teacher from doing that and limit her homework to no more than 1 hour per day, for instance. I'm happy with the amount of homework myself, as is my son. However, if there were to be any evaluations in that class, the teacher would be knocked down in no time.
What this leaves us teachers with is a bit of an "unmentionable" problem and its consequences in private institutions which your TEFL course won't likely go into in very much depth, but which you will end up finding a serious pain to deal with nonetheless.
Learn how to deal with mixed-ability classes
What to do with classrooms a quarter-full of students who are clearly out of their depth (otherwise known as mixed-ability classrooms) or above their level? In my experience, I've seen this problem in many language programmes (even in universities) where students are just automatically moved up a level every year regardless of their real current level. In the end, they may even get certificates from your institution stating that they have a far higher level than they actually have, and you may be the one who has to sign them.
Sometimes, for instance, the school you'll be working at will have nowhere to place the three or four absolute beginners who have enrolled in the school (not enough for a full class maybe?), so they'll place them in your elementary class. The levels may sound similar, but they're not. In my opinion, beginners have no place in an elementary class, so good luck teaching them and dealing with the fallout. These types of situations are common enough, but try to imagine having the beginners who fail these elementary classes and their elementary classmates be obliged to skip over an entire pre-intermediate level in order to place them in an intermediate-level class for bureaucratic reasons beyond comprehension. Then, imagine those students going through two more years of a situational/functional course such as English for Medicine, English for Business or English for Technology. Finally, imagine yourself perhaps having to sign a paper stating that all of the students who pass your English for Technology course have a B2 level according to the Common European Framework for Languages (because you'll have to pass them despite the fact that they probably won't really have the level unless you want to become a victim of the fallout from your own evaluations). This sort of thing really happens, believe it or not!
Maybe the long-term problem doesn't worry you, but in the short-term what this means is that your students won't have the level to understand many of your explanations. For example, everything that an absolute beginner learns in a year is reviewed within the first couple of weeks of elementary level and then it's on to new topics and grammar points. It'll take beginners a week just to learn how to pronounce "What's your name?" and "My name's . . ." An absolute beginner will be blown away for the first two weeks of an elementary class and will be totally lost after that. So, why are they being placed there?
TEFL courses may choose to pretend this problem doesn't exist, but it does, and you need to know how to handle it. Rather, there is no honest way for you to handle it except suck it up. You'll have to look the other way on the one hand and handle it as well as possible on the other. Whether in bigger or smaller institutions you basically might get a coursebook that maybe a quarter of the students in the course can't handle outright in the beginning and good luck with that.
The correct way to handle it, I believe, is to teach very lightly and carefully (don't be too demanding) then make sure the exam only contains material that you've covered very well in class. For example, there might be 1,000 important new vocabulary words in the coursebook, but you should occasionally play vocabulary games with maybe 100-150 of them so that they sink in better, and then put these on the exam, not the other 850 which they obviously won't have studied. The same goes for grammar. You can cover more grammar points in your teaching as it's expected in Spanish classrooms, but try not to nail them too often with the finer points of grammar on your exams.
Learn how to "administer" your classes (get your feet wet)
I've found "administration" to be really important when teaching in institutions. Especially important is how to prepare exams and I think it ought to be on TEFL courses, but this probably won't be on them either.
In one job I had, for example, I had 5 separate courses and had to put together 5 mid-terms and 5 final exams every year. (The first year I had to also put together 5 practice exams for the students, not for their sake, but for mine. I didn't want to inadvertently prep exams that were too difficult and go and accidently fail everyone. The profile of the typical student was pretty mediocre. It took a little work to get it just right.) All the students who failed any exams got to almost immediately take a make-up exam for each failed exam, making that a total of 25 exams to begin with, even though ostensibly I wouldn't have had to have done it except that some of my students never ever showed up to a single class, and had a right to take this extra make-up exam. But, wait, it doesn't stop there, because students in this school had a right to take up to a total of 5 make-up exams and I had to prep every last one of them - each one of them different from the last. So, absurdly enough, I had students take two separate make-up exams within a two-week period. If you fail many students, imagine where you'll be in your third year if they don't tar and feather you first: up to your eyeballs in exam prepping. The best way to avoid all this is to pass everyone, hopefully because they deserve it, but methinks that there are other ways students are passed. (Recall the NYU professor's "never again".) (Also, doesn't this school seem just totally DESPERATE to get students to pass at any cost? The students only needed a "5" to pass for Pete's sake!! And we even had instructions not to give anyone a 4.7-4.99 so the management/DOS wouldn't have to deal with irrate students/parents.)
In addition to all this in this particular job, there was all the other administrative work to take care of, attendance, homework and quizzes - and loads and loads of corrections. This all takes up so much time and energy that I am really awed by teachers who (say they) have the time and energy to design their own lessons, materials, etc. Are they familyless/friendless celibate monks? Give up all hope ye who work for such red-tape-heavy institutions and want to moonlight as artists/photographers/musicians, etc.
Learn how to prepare your own materials (and good luck with doing so on the job later on)
And, yes, TEFL courses should teach you how to produce some of your own materials. Specifically, in my opinion, there's a dearth of good listening activities in most coursebooks. Also, in most institutions I've worked in, the school's idea of "support" was to tell me that I could go and find materials on the internet. The last time I asked for materials, the process was such a difficult and time-consumming run-around that I soon forgot about it. I couldn't even get a straight answer for months and when the response did come through, it included processing forms to be included in a request to be included in the next year's annual budget proposal for some other department which was only processed once a year.
Be aware though that although the lessons you've created might be motivating and fun, "minimalist" students who favour materials that are clearly going to be showing up on any exams may not like them unless you simply state that the vocabulary, etc. will be on the exam (it doesn't matter if it's a lie). Also, if you do have this vocabulary on an exam, some students who miss this particular class (one or two always will) will then request materials that they can study for homework so as to do better on the exam. In my opinion, fun materials are best employed immediately before long holiday weekends when most students traditionally disappear from the classrooms anyway. Then, don't have it on the exam. But, go back again and check out the bit about deciding how much of your class should be "work" and how much should be "play". When you sort that out, let me know, because I'm still stuck on the fact that class should be about "work".
Also, be especially aware that most "realia" as it's called, or material you find in the real world (Youtube videos, songs, etc.) is going to somehow be too difficult or inappropriate for your students in your classroom. Your TEFL course will likely spend more time on "realia" simply due to the fact that it'll help you to understand the whole lesson prepping process that much better, as well as learn how to exploit the internet which is where your DOS will send you to look for materials when you ask for some (remember?). This is the crux of another TEFL conundrum: what is the best balance between textbook and other realia materials or other supposedly "fun" non-textbook activities, for that matter?
Similarly, you should want to learn a bit about resource books that are available for English teachers. Hopefully, your TEFL school will have a large selection of them for you to browse at your leisure without the presence of an unpleasant "growler" watching over them, "Yea, you can have a look at them, but I've got my eyes on you." Basically, my impression is that many schools really worry about their teachers either stealing their resource books or losing them. Sorry to bring that up, but it seems to be a relevant tip around here.
Learn how to manage classrooms
Of course, the main things that a TEFL course ought to teach are things such as, obviously, how to teach, correct, manage classrooms of students, put them in pairs to do pairwork or groups to do work in groups.
Presentation, Practice and Production (PPP), within the framework of the Communicative Approach, ought to be enough but more and more often, it isn't. Spanish students nowadays tend to be a bit spoilt like so many others in the west. They expect teachers to all be as entertaining as the television programs they boob out on which have been carefully prepared and edited. And, above all, they too often expect to be able to sit passively and participate little (inactively) and gain the same levels as their more active though far less numerous peers who participate, work, study and memorize.
Learn about Spanish students
Also, TEFL courses should help you a bit with how to motivate your Spanish students of English and/or how to get the them to behave. For example, many Spanish students tend to be too talkative in the classroom. They interpret the more open nature of the TEFL classroom as an opportunity to let it all hang out. In fact, it's always, always, always the worst students who'll make you do the most work. (remember all my make-up exams?) The best way to avoid doing too much work is to have good students who do most of their homework and classwork all by themselves. (Good luck with that plan though!)
There are plenty of other things that you will have to learn after your course. The devil's in the details, as they say. There's no substitute for experience for learning about things like how the system, bosses, students, parents, other teachers, etc. mess with your head and life.
Do you really need a TEFL course? (three possibilities):
ABSOLUTELY! . . . if you want to teach in private academies, schools, universities, etc. To put it bluntly, all private institutions will want to separate the men from the boys. They will all want to make sure you know the basics of what you're doing and they will have other teachers with these certificates to choose from if you don't have a certificate. 
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