Fruit, all the way. When I know I'll spend the entire day at school - morning and evening classes, regardless of what is in between - I'll take (at the very least) a bag of apples. Any fruit will do but apples are usually the cheapest ^___^
A bunch of people have suggested coffee and energy drinks, but I find both of those make me
very thirsty. Also, when I'm dehydrated I lose my focus very easily. I suppose I should also add water up there. I usually consume at least 3-4 water bottles (of water) in a normal 9-6 school day.
Also, snack food like candy bars and chips/pretzels/trail mix make me feel full and sleepy. I usually nod off very fast after I eat something at the uni
How ya doing, buddy?