I'll take the minority position, that the school officials did the right thing.
If they have a clothing policy for students, then it should be enforced. Even if there is no written policy in place, I don't think it'* appropriate dress for that kind of event.
The argument that it'* no different than one of the guys doing it is rather inaccurate. A bare-chested man is just a bit more socially acceptable than a bare-chested woman.
Well, with the exception of all the posters above me...