At what point does control over a student become an infringement of rights?

My school (an independent co-educational boarding school in England for ages 13 to 18) has enacted a supposed "ban" on short shorts and skirts (even with tights), patterned tights with 'suggestive patterns', low slung tops, any tops showing bra strap. This of course only applies to female students. For the entire school day we were a kilt (below knee length) and a formal white blouse with a blazer. In our free time, we are surely entitled to dress within the boundaries set by society and English Law, which would thus suggest that "short skirts" are not a display of public indecency. To me this seems to violate the fundamental rights of young women and is oppressive on various levels. I am 16 and the fact that I should hide the reality that I wear a bra and have the right to present myself in a way which society, not a school board, sees fit. It's not 1953, it's 2011... surely are society has advanced enough to accept the rights of young women rather than to shun them. At an age where it is perfectly legal to have sex, it seems rather prosaic that older generations still view the female body and sexuality as a threat. Is there any legal case to be made of this?