Excellent points crim, and if I may add something to your mental practice thing... you're lying in bed or in the car or almost sleeping during english class, so what can you do? Why not do some finger excercices, they are very valuable. Try pressing all you fingertips against the palm of your hand, then shooting them out, one after another. Then try the same thing with your fingers pressed against a hard sufrace, only you're just shooting out the middle and index fingers in an alternating, "walking" type of way. This one is great for classical guitar players or bass players who rely on the alternation of the middle and index fingers as their method of alternate picking.

Those are only a couple, there are a lot more you can do.