Drink plenty of water all day, every day. Also drink at least one bottle of Gatorade as it replenishes your body with sodium, electrolytes and other nutrients your body needs. Stay away from soda, caffeinated drinks and any other sugary drinks as they will dehydrate you.
Keep yourself hydrated (you'll probably need more bathroom breaks, so I'd suggest using the bathroom between each class just to be safe).Wear thinner, looser clothing (trust me, a body-hugging tank top makes you sweat like mad). Wear unscented deoderant, you don't want the person sitting next to you to pass out each time you raise your hand. And you can wash your face and shoulders a bit while taking a bathroom break. FINALLY, take a shower after school, a nice cool one to keep your skin hydrated (do NOT take a freezing cold shower, just take one that is a bit cooler than you're used to).