If I had to take a wild guess, it would be that they're trying to prevent the boys from dressing like they just rolled out of bed and the girls from dressing like hookers...

I'm 22, so I haven't been so far removed from schools for long enough to forget middle school. And it seems like every year young girls dress more and more promiscuously...least that's my take on it--and also the reason I NEVER want to have children. Toooo scary.

EDIT~ I am -not- accusing your girl of dressing promiscuously, either. It just seems to be a trend going on in society at the time being.