How can I become skinnier?

I am 13, and just the right weight. I have been told by doctors, computer things, and BMI charts. I am also 5'5, but I still have a belly. I am a vegetarian and I rarely eat junk food. I exercise,too. People who are shorter then me, or even taller then me, eat a lot of junk food suck as school lunches everyday, fries,tacos,pop tarts, cookies, brownies layered with powdered sugar, and chips. I don't eat that, but everyone else does. All of the people that eat unhealthy are way more skinnier then me and have a bikini body. So why do I have a belly and how can I get rid of it?!?