You can not compare her with a boy being in the "girls" locker room because she is a girl and there are a few physical differences.
As a mature man, I work with some very attractive women. Yet I manage to make throught the work day without drooling all over them. Just because she is attracted to girls, does not mean that she lusts after every girl she sees. Chances are, she is not sexually attracted to any of the girls in the class.
Lastly, lets remember that she is only 15 and is having to deal with alot of conflicting emotions. I know I was at 15. I'm 48 now and I still can't figure out the female mind. I don't think any man ever will. I say "She is a little girl, so treat her like one until someone complains that she made improper advances".