I would stay away from dairy.
I have a young 16 year old cousin and From what i see the young girls doing these days (and i will repeat some stuff dryvrgrl says) as far as potential reasons for weight gain:

1. Fast Food (Eliminate it period)
2. Alcohol (Binge drinking on weekends +tons of extra calories)
3. Fancy COffee DRinks (Some of these monsters are loaded with 1000+calories in one sitting)
4. "Healthy" Fruit Smoothies (All these Jugo juice and booster juice places are loaded with carbs and sugars.
5. Not enough Meals (You want 5-6 small evenly divided (in terms of protein, fat, carbs) meals spread out evenly throughout the day. Will keep metabolism reved instead of skipping breakfast, pigging out on maybe 1 meal of the day.
6. Not enough sleep. Kids are busier than ever now adays with having school, part time jobs, social life.
7. Late night Binging: dont let yourself be awake at 3am with friends and ordering pizza's or going through McD's drive through
8. Not enough Water
9. Watch your condiments : Sauces, additives...etc all come with a price.

I still would reccomend you figure out something you can see yourself following as far as a basic diet guideline....write it up here and we will help you with it...doesnt have to be 100% strict like a bodybuilder (as we understand you are only 16) but give it a try.