Classes can be overwhelming at times. It's now 2:41am and I have class at 8am. of course I'm in college so the teachers don't care what work gets done and what work doesn't get done.
It all takes planning and the motivation to do it. Just pace yourself and get into your studies. You have to do it, may as well be planned to do it. It'll stress you out and if you have troubles staying focused, think of the things that distract you and shut it out for the time while you study.
High school was rough for me, as well as college as well, there are always those kids that "get it" and we're stuck with "uhh..."
Hang in there man,