I know how you feel...my classes start at 6 am and I get home from gym at 9 pm...basically I use all of my free time to get homework done. If you sit in the back of an easier class, do homework. This is my #1 tip, sorry parents and teachers. My #2 tip is to spend your lunch time doing work, especially if you can use computers in the library during that time. You can eat while you read, or perhaps one of your teachers will let you eat an apple or something non-distracting in class. Do homework in the mornings before school. Study like a madwoman on the nights you don't have practice and on weekends.
Basically just front-load majorly. If you get it all done ahead of time, like on a weekend, then you don't need to feel stressed. Plan out essays ahead of time, like if you're bored in class or in the car. Also, you may want to take only the SAT. Almost all colleges accept the SAT, but not all take the ACT. For AP classes, keep all your notes and review periodically. Doing the reading every night is probably the most important de-stressing tip - a single reading generally isn't too much, but if you have to catch up, it's overwhelming. Review books and online quizzes really help. Also, remember that school comes FIRST - before gym and before friends. I can't count how many times I've turned down an offer to go to a party because I had a bunch of projects due.