I have seen enough teachers do some incredibly stupid things to know they aren't infallible. It's pretty clear to me that the school system doesn't always have a monopoly on insight, at least on social issues that effect school administration. And I do think there is an element of teachers and administrators that takes in loco parentis entirely too far.
I think there is a time and a place for everything, and a freaking prom should be a little more lax. It's not an educational event, it's a social one A) Because evening wear is by its very nature more revealing. B) students actively organize and pay for the prom, and in many instances, while its a school function, its not on school grounds. Granted, I know schools have "jurisdiction" still, but as an optional, social event, they should back off a bit. It's not like they can argue that it is interfering with the lessons or day-to-day administration of the school. And I always find this stuff funny--no one gives a damn about the obscene amount of conspicuous consumption that goes into the prom, just if someone can see a little more skin than usual. Never mind that you can see tons of skin on "family shows" such as Dancing with the Stars or Figure Skating...
The policy should have been in place beforehand, and that's that.
The opening scene of the movie "Saving Private Ryan" is loosely based on games of dodgeball Brian Dawkins played in second grade.