One thing that's important to remember is that people are teasing you because they feel insecure themselves, and making fun of you probably makes them feel better. This is an annoying fact of life. People are really weird about body and size issues especially when they are in the seventh grade when everyone's body is changing so drastically. I had the opposite problem of being so tall so I asked my friend Rebecca and she said:

I was one of the smallest kids in my class until high school, and I was teased all the time about being flat and obviously not as far along on the puberty train as everyone else. I felt like I would never catch up to the way a teenage girl's body was "supposed to look," and being teased about it just rubbed it in. When everybody grows up and calms down a little (which they will, because where you are right now could be considered the max-out point of bra-cup size obsession), they will stop teasing you about it so much. In the meantime, try to relax and think about how stupid they are.