hello there. If you were in your shoes I would probably start focusing on the little things such as playing a sport whenever available like say.. The P. E. Activities given at your school. You could always go biking, swimming,  or running afterschool(it's summer). If you would like to slim down I'd advice eating right and not buying chips and other junk food despite school food tasting extremely bad. The best alternative is to bring home cooked food to school and drink plenty of water. You could also join a local recreational gym to help you with your goal with say..  Boxing or even weightlifting. Good luck, Highschool is approaching. Earn the Body you want before then, Cheers!