Have you thought about getting your GED? Maybe the reason why you're unmotivated by schoolwork is that it is useless to you.

What types of activities do you find difficult to do? Cleaning toilets, doing math problems, writing reports?

One technique that I use is stolen from PJE (http://dirtsimple.org/2006/01/smelling-fear-feeling-future.h...), The "Mmmmm" test.

Generally you're doing these things to get something else that you find worthwhile. If you can visualize this goal deeply enough to change your emotional state, like Homer Simpson fantasizing about donuts, then you can associate it with the thought of doing the drudgery as well. It takes time and effort to retrain yourself to do this, though.