If a girl doesn't want to be grinded on - then only go out during certain songs, stay to the outer calmer fringes of the floor or don't go to the dance. The school has no right to limit this as long as the students rights of privacy and not being inappropriately touched (to a point of sexual assault of the I'm thinking it's 3rd, but it might be 2nd degree).
Inks has it on the money, here, surprisingly.
And you think this school's policy was bad - my (private) school thought dancing was a "sin" - a bunch of b.s. - there's dancing in the Bible. Not that I was sad - I can't dance at all and will probably refuse to at my wedding as well (harsh, but if you've seen me dance, you'd know why).