Eek, yeah you don't want to end up going into your exams thinking only of your story and not the things you need to be remembering!! Are they at the beginning or the end of the month? If it's the beginning you could always just start after your exams finish and see how you do? If they're at the end though I'd probably focus on the exams, you can always try doing it in December unofficially after all
Well, I actually hit a point where schoolwork and emotional issues made it so I couldn't write, and I ended up stopping at about 30,000. I think that's a good thing, though- now I can say I've been on both sides as far as winning or losing NaNowriMo goes.