weight loss mainly comes from your diet so just eat healthy and your onto the right path, ask for help from any health advisors you have at your school, theres normaly someone on hand there.
Try to eat 5 portions of fresh fruit and steamed vegetables (potatos, parsnips and anything battered do not count) and if you can add to that great.
Eat protein (meat, fish, eggs, nuts, tofu..etc) 1 portion per meal, try to steam or grill rather then fry, frying adds fat and too much heated fat is not good for you as the fats are not useful to the body (unheated oils like olive oils, flax oil or even omega 3 oils are better but be careful not to have too much).
Eat complex carbs, one to one and a half fist size per portion, 3 times a day.
Drink plenty of water but do not force yourself to drink more then you need, its about listening to your body. Try to drink water over diet drinks, try not to drink full sugar drinks.
Obvious things to avoid would be sugary things like sweets, refined carbs like white bread (as oppose to wholegrain bread, brown bread is just white bread dyed brown btw) cakes, pastrys, crisps, tortella chips, white rice...etc nothing you don't already know I'm sure. Your food intake makes more of a difference then your exercise regime although exercise can help you alot.
