From what you've said it looks like you have a fair amount of time to prepare your students. You can achieve a lot in 25 weeks with 4 or 5 classes a week. So you have the time to do a fairly thorough preparation.
Having said that, it is still very important to assess where your students are starting from. By this I don't just mean their overall English level, but their specific strengths and weaknesses in terms of individual TOEFL question types.
So a good starting point is a diagnostic test. Many preparation text books have these - one I've used and liked in the past is the "Cambridge Preparation for the TOEFL Test". The diagnostic test key in this book tells you exactly which sections you need to work on with your students if they get a particular question wrong. As you go through everyone's test (and it's worth spending the time on this), you'll build up a very accurate picture of strengths and weaknesses in terms of specific question types, which you can then work on.
What I usually do then is start with the area of greatest weakness and work down the list until I run out of time! (If you have limited time, you have to be selective in this way).
As Lucy said, exam skills are as important, if not more so, than imporiving their general English level. The TOEFL is as much a test of knowing how to recognise and answer a particular type of question, as it is a test of English! And this is what you should focus on in your class time. Again, a good preparation book will give you a guide on how to teach your students the skills needed to answer a particular type of question. This could be identifying a suitable title for a text, understanding words in context, drawing conclusions from a text, and so on - these are skills that can be taught and practised again and again.
You are right that it is very important for students to work outside the classroom, reading, writing, practising and practising some more.
I hope this helps - good luck!