2-4 slices whole wheat bread
2-4 tbsp natty pb
1 apple
It can get a little messy w/ the natty pb, but it's so much better than the regular pb. It's simple, doesn't need to be refrigerated, and the complex carbs will stay with you throughout your workout.
Or you could do 4 slices wheat bread and some type of lean meat if you wanted more protein and less fat. Maybe those packages (not cans) of tuna. It'd be much easier to deal with at school instead of always having to carry a can opener.