Be prepared, specifically in two ways: 1) Go to bed early and get a solid 8-10 hours of rest each night. This way, there's no chance that you'll fall asleep in class. 2) Do your homework and keep up with class. This way, you'll likely be more interested and more engaged while you're in class.

I heard this tip from a friend. Part of feeling lazy or uninterested in class is due to the fact that students are mindful of the time. If you see the clock and you realize that you're going to be here for another hour, then that'll turn you off. Who hasn't sat there and basically watched the minute hand tick away until the class bell rings? A good tip is to avoid looking at that clock and get rid of all watches, cell phones (although you probably shouldn't have cell phones while at school), and other timepieces that you may have on you. This way, your only focus will be on the class itself.

Stay awake. :]

To focus better, try keeping an agenda or a planner. That way you can stay organized and not get stressed out about the homework. Not only that, however tempting it might be to sit by your friends in the class and talk, RESTRAIN YOURSELF. Also try taking notes in the class. Taking notes will keep you on task.