No I think that at you age you shouldn't have any risk, but exercise its not only for physical health purposes but mental too. What I mean is that helps you to deal with the strees that you have at school. My suggestion, have a couple of basketball, tennis, soccer game a week, or walk when you can even if is just a couple of miles a week, eat properly, swim, play bowling, walk you dog twice a day or just go up and down in your stairs at home or at school, you will feel much relax and without the worries that you have now. Good luck, hope that it helps.