I second ducky's tip to bring your own lunch. That would be a MAJOR detractor in trying to lose weight. I mean, shoot, they are trying to get pizza PIZZA! to count as a vegetable in school lunches.
Definitely exercise. If you can't join a sport after school see if there's a gym nearby (planet fitness is having a deal right now and they're in a lot of cities)
If you have cable television like Comcast there are a lot of exercises you can follow on there and do from home
Good luck!
Well first- eat breakfast. Eating at the correct times to prevent yourself from gorging really makes a huge difference. So make sure you eat breakfast, lunch, and dinner and try to throw in a small low calorie snack after school and maybe in between classes if you can. You need to make sure you're eating the right amount of calories for your height, weight, sex, and age there are several sites that could give you a little direction on that and if you are obese many medical clinics allow for insurance to cover seeing a dietician on the recommendation of your doctor. That might be able to help point you in a good direction.
Also, I know there has been many a change in school dining, but my experiences were pretty bad. I'd suggest bringing your own lunch to school. That can make it easier to know that you're obtaining the correct amount of calories.
Also I know it's the age for it, but back off on the soda, chips, sugary snacks and really as much processed foods as you can possibly do. I lost a quick 10 lbs from just switching my drink to water and my foods to homemade- with none containing high fructose corn syrup.
For dinner try to fill half your plate with veggies. Make sure you're not loading on cheese, dips, or tons of starchy (potato and the like) kind of veggies. Good leafy greens, carrots, and fruits are the way to go.
Exercise is also needed to help start your metabolism. Try joining an after school activity. No one expects you to be amazing and even if you're on a sucky team it could be a lot of fun! I know after school sports can get pricey so if that's not possible try walking to/from school or at least from a further bus stop. You need to try to work in about 30-1 hour of high activity. (Really you just need to get your heart pumping.) If you have a dog, now is a good time to teach him/her to heal :D