Make small changes - have a glass of skim milk or water instead of soda; replace fast food meals w/ a turkey sandwich on WW bread; have a salad everyday before supper, and instead of dressing, squeeze some fresh lemon juice on it.....
Keep eating - but healthier - and you will lose weight. Try to be more active than you are, too - maybe go for a walk after school or after supper. You'll be surprised at how little changes make big differences.
I don't know what your current diet is like, so if you want more specific advice, maybe you can post it?