You're on the right track! The Academy sites mention somewhere that a good foundation in Algebra is better than exposure to Calculus. However, you answered your own question--take the summer class too. In fact they like it if you take a computer science or prog. class as well.
Lots of H.S. activities, volunteerism, and leadership roles. Push yourself to extend your leadership capabilities. Stay focused. Prepare for your interviews with the Liason Officers and the Nomination committe by thinking of dozens of questions they might ask you and practice answering them. Have a smart friend listen and critique. Tape yourself.
Most importantly be yourself and let your answers reflect how you really feel, not how someone thinks you should feel or answer. Be passionate and clear.
Character matters--make sure yours is squeaky clean and that you are an honorable person--read and memorize the Air Force Code of Honor.
Good luck!