I care more about weight loss than school/alevelsWatch
That being said, a HEALTHY attitude to exercise is a good thing so I would say look at how you can get balance in your life. I'm a big believer we can have it all (you can be successful, have a good social life, feel good and confident although not necessarily in the perfect fantasy way we initially think). I hate to encourage restrictive eating and i am NOT saying revise to avoid food, but structure your days so you get a routine. School and revision are times to focus on that work, not sit and snack or walk around. Time away from that is time to exercise and fuel your body in a healthy way.
If you feel like the thoughts of food and exercising are taking over too much then please talk to someone- you're absolutely right in that you have your whole life to think about your body but your a-levels are now so take care of yourself!