I'm also 13 and I lost 4lbs in a week by just watching my diet:) no junk and only drink tea, coffee, and water... Mabe brink a healthy snack to eat at school cuz eating small portions more frequently is better than eating few large portions.. But always eat cuz you could go into starvation mode and just gain more. Hope this helped:)
Don't stick to a diet just eat healthy and not too much. If you try sticking with a diet then you have the pressure of staying with it and the more planned out it is the more bored you get of it. Just eat healthy and avoid fatty foods and junk food and you'll get there without even needing a diet.