Ok, heres the deal, work hard in gym class. If you play hard its fun. If you have fun others have fun. You make new friends. Yay. BUT always keep your eye on the clock... stop working hard a good 15 minutes before the period ends [10 min or so before you go to change]. That way your body will be back @ normal temp by the time you have to change/go to class, and you won't be sweaty.
Also, use UNSCENTED deoderant [arm and hammer works great], and a LIGHT spritz of your calogne. It will never clash, and it won't be obvious your covering up.
Always bring a new shirt to wear in gym. The tighter the better. It will absorb the sweat so when you change out of it you won't get your new shirt all salty.
Uhhh, thats all I could think of.