If you faint everytime after gym, then you need to go to the doctor and find out why. If your doctor already knows why then he can write a letter that you can take to school to allow you to sit out of physical activity. And that doesn't even require a doctor's appointment. Your doctor should be able to fax it to your school.
I was born with a heart valve abnormality but it didn't and still does not effect my normal daily activities. However, gym is strenuous, and I almost blacked out after running for class in 85 degree heat. My doctor promptly wrote a letter saying that I could at any time sit out. My school felt it was too risky and forced me to sit out whether I liked it or not. Sure, I was a little saddened that I couldn't play sports with my friends, but my health was more important. I couldn't receive an A in gym class, but I was happy with my B+ for doing nothing more than stretching and a few written exams. They cannot discriminate against you for health problems. Your profile and user name hints that you may also suffer from depression which can be an underlying symptom of a greater health issue. Many times people do not realize how often the mind reflects physical inbalance.
Bottom line: Magick should not be used to combat misuse of your body. Pushing yourself further than your physical limits while convincing yourself mentally that you can may land you in a hospital, or worse a body bag. Please don't. Lack of energy is not the reason you are fainting therefore you are trying to cure the symptom and putting additional stress on the problem.