Yep, no crash diets. They don't work, and they're really bad for growing bodies.
The very best thing you could do would be to talk to your doctor or school nurse.
In the meantime, instead of saying that you're trying to "lose weight," a much healthier approach is to tell yourself that you're going to only put good things into your body to keep it healthy and fuel your workouts. Imagine your body like a car. If you put junk into the car, it won't run well-- or at all!! If you put high-quality fuel into your "car," it will perform really well.
So, what makes your body perform? Fruits, veggies, lean proteins, complex carbohydrates. Lean proteins are: lean meats, fish, yogurt, lentils, beans, soy milk, eggs. A hamburger does not count, neither does friend chicken.
If it leaves a grease spot, don't eat it!
Complex carbohydrates: whole grain bread, pastas. A good rule of thumb is, "If it's white, it's not all right." White breads and pastas are bleached, and there is very little nutritional value in them. So, "If it's brown, eat it down." (Ha, I'm a dork...sorry.)
Salads are a great choice, as long as you avoid creamy dressings. Did you know that some salads with creamy dressings (like ranch), have more fat and calories than a greasy hamburger?? Ugh.
You can't go wrong with whole fruits and veggies.
Eat things like cheese in moderation. Try to avoid fried foods and "junk," like ice cream, candy, donuts, etc. Pay attention to the nutritional information on the box. Look at the serving size and just eat that much. Also look at the ingredients list. Complex carbs (like bread) should start with "whole grained wheat" at the top. Avoid "high fructose corn syrup" (a highly processed form of sugar) and sugar.
You'll be feeling lots better in no time. Again, talk to your pediatrician or school nurse to make sure you're eating well, but in the meantime, you can start with these tips.