I gained weight my first semester, but that was because I was on a sports team and was working out and eating with people twice my size.
When I quit, I went back to about 5 pounds above my HS height. I walk considerably more at school than I do at home, and I eat healthier, so I imagine that has something to do with it.
HS girls are all just really skinny. Last summer, we worked with HS seniors, and I forgot how skinny they can be. And they don't think they are!
I also agree with OULioness's theory.
Although, part of the reason I got on BC was because I (and my doctor) thought it would add a few pounds on me.