My goodness! So many generalities in one thread!
Of course students have rights and the legal profession can help to protect them. But I doubt that will be necessary in this case.
IANAL, but I was a public high school teacher in a language arts department and certified to teach dramatics. I cannot image that this teacher could get very far with this rule as it is obviously not in the students’ best interests.
The first step would be for a student to request a conference with the teacher to ask for an explanation or for reconsideration. Then the next step would be to bring in parents to talk with the teacher or with the principal. These things should go through channels and it doesn’t take as long as you might think. Principals do not want any bad publicity for their schools.
I think it’s great that you and your friends have found such fulfillment through the theater. It’s a natural high!
Break a leg!