I had this problem, well I was a twig until grade 7 when I started gaining weight, I was my biggest in grade 11 then over the summer before grade 12 I started to loose it again, finally I am thin again but I still have to watch what I eat. oh another thing whenever possable walk, I lost a lot of weight because I almost never set foot in a car, I either walk or ride my bike everywhere possable, I believe this is a huge part of it. another thing I tried that worked great it to eat something small every 3 hours, like every 3 hours eat an apple or a granola bar or something, that way your starvation mechnism never has the chance to start so your body will not hold the fat from your meals.