Ah, what has happened to the days where real young kids(10-15) never had this problem. No insult to you my good man. I mean i remember when i was 13(only 5 years ago LOL) i was playing football and baseball and i was playong them 5 days a week like all year. That alone was what kept me in good shape, plus that 13year old metabolism
Dude, my 2 cents is to stop doing 9,000 sit ups a day and start just being active. Go try out for football or something. You weight training class is a step in the right direction but a lot of "school" weight classes are crap.
Yesh ur diet probly isnt good, thats the hardest part bit there are allot of stickys floating around here devoted to that.