Do you have a smart phone? You can use calorie counters like myfitnesspal that log you food intake to see how much you're eating.
Definitely bring your own lunch to school! It can be a hassle, but school lunches are usually high in bad fats, and high in sodium with few-to-no fruits/veggies.
Also read labels on foods! Just because something is listed as "healthy," "fresh," or "all-natural," doesn't always mean they actually are healthier. Food companies put these labels on to up their sales, and don't care about the consumer's health.
Are you able to exercise at home? Maybe get an aerobics DVD? Walk your dog?
1. Miria Harvent, Baccano! (and Isaac cosplay for my fiance)
2. Raava, Legend of Korra (Human form)