There is a privacy law called FERPA that makes video of classroom activities very complicated. First yes you are right that when someone walks out on stage their rights to privacy mostly go away. But if the performance is for a class and graded. Then the video recording could potentially be called a record of their education, which FERPA says you can't do anything with. It gets really complicated as to if you are saving it as a record of their education and then worse identifying who is in that record of education. Theater/Music performances get more complicated than that depending on if the public is invited or not and if you allow the public to record or not. FERPA being a well intentioned government document that really was designed to make sure a teacher doesn't post videos of Johnny screwing up a hilarious speech in history class, doesn't realistically consider performing arts. It's complex and full of what I think of as contradictions. A lawyer could make your life a nightmare with a lawsuit even if you did win in the end. So the easiest solution is to have a release before you as an employee of the school record any school student on stage.I think a school is a public place, at least there is no traditional "expectation of privacy" in a classroom or auditorium... but because we're usually taking about minors there is a whole lot of hand-wringing about student privacy.