Get enough sleep! Kids and teens need 10-12 hours of sleep every night in order to feel alert and smart in school.
Eat the right foods! Stay away from junk food, because it gives you a "sugar rush" that will wear off after 15-20 minutes and then leave you feeling even more tired than before.
Stay away from "energy drinks!" First, they wear off after 15-20 minutes and leave you more tired than before -- even the "Five Hour" drinks will "drop" you once they wear off. Second, they have massive amounts of sugar, which is bad for you. Third, they are very hard on the stomach -- I've seen kids who developed ulcers from drinking too many of those things (I work in a pharmacy).
Exercise! Your body actually feels more alert right after you exercise than it does if you don't exercise at all -- this sounds backwards, but the body needs movement in order to function properly.
Breathe! Taking deep breaths helps get more oxygen to the brain, and makes you feel more alert -- this is why we yawn.
Drink a glass of water! I know, it sounds weird, but it does wake you up, especially if it's cold water.