I'm 12 years old and i need help losing weight- and fast! Please help!!!!?

Im 12 years old and i weigh about 170lbs. I really want to lose weight. Its hard to look at the skinny girls at school and think, i am so fat. I have been pestering my parents to let me join a gym but they wont let me. I want to lose as much weight as i can before school starts, and ive got 2 weeks till school ends. So pretty much im on my own with this. I can eat pretty healthy at school, they offer lots of fruits and veggies and salads, but i cant once school ends. its also the exercising part. i do have P.E. every other day but not for long. And i cant just take off and say im going for a walk. so i have to do some kind of exercising. What are the best exercises for me to do??? I also swim like once a week during summer. Please help. Im not looking to be a super skinny girl, i just want to be healthy.