At 204 pounds, you aren't fat but perhaps you could lose a few pounds and feel better for it. This would be an excellent way of stopping the bullies in their tracks too.
I am concerned by the fact that you are getting beaten up though, have you told a teacher about this? It is something that you certainly shouldn't have to put up with, so get that dealt with as soon as possible.
Newsflash: Popular girls are nasty by nature.
I hear things almost every day from my teenage daughter about the so called 'popular girls' and the more I hear, the less I like them. Ignore these people, they really are not worth your time, they think it's cool to bully people, but they will learn the hard way one day.
As for your weight, it should be easy to lose a couple of pounds each week until you feel about right. Exercise is a brilliant way to burn calories, all you need to do is go for a brisk walk for half an hour a day, alternatively try swimming or cycling.
It won't happen overnight, but if you stick to the same routine, in a few months you will begin to notice some major changes.
You will also need to cut down on the things that you eat which you know aren't too healthy. Most people have a weakness for something, whether it is cakes, chocolate, crisps, sugary drinks etc... Try to cut these weaknesses out as much as possible, and aim to eat fruit in place of them, as it will make you feel fuller.
Please talk to an adult about the bullying though, this needs to be stamped out right away. If you want to lose a bit of weight for your own peace of mind or health, then that's great - but if physical violence is taking place, then these kids have crossed a line and deserve to be reported to the authorities.