How to lose weight without your parents knowing.?

Okay, I'm not trying to get anorexic. Let me just say that now. The thing is, I want to tone up, but I don't want my parents knowing because believe it or not, they'll tease me about it. They already comment on the little bit of fat I have, hell my brother's nicknamed me manatee. So I do want to lose weight, but I don't want them to know it, because I have a feeling they'll hold it over me. I would go to the gym if I could, but you have to be 18 to get a pass here. I'm 16 by the way, 5foot3 and about 115-120lbs. Far from overweight. I just want to tone up, without them finding out. Any Ideas?