Lets make a reasonable comparison.
Most of the schools allow students to wear pretty much what they want but when it comes to being braless especially if it is obvious they draw the line.
They are however allowed to wear their gang banger attire and make it obvious that they are gang members and their duty as a member is to recruit new members. They torment and abuse other students using fear to convince many great kids to join their gang. Then the violence and drugs start.
I think they are out of focus with the real problems. A girl that is braless is much easier to control than an out of control gang. Teachers and faculty are scared for their lives by these gangs and choose to ignore them.
Maybe they need to take a hard look at the real problems...