At least in the United States, most schools are not a place of privacy as the schools have a certain right to authority over their pupils. Consider Tinker v. Des Moines and what it meant for freedom of speech in schools. That case won students the right to freedom of expression. It’s important, but in certain cases it becomes limited by Morse v. Frederick, a case that ultimately meant that such expression must not disrupt the learning environment. All of this is to say that students have certain freedoms until expressing those freedoms is disruptive to the learning experience, and I don’t think there’s any solid argument that would not consider vaping disruptive to the learning environment. Considering this as an invasion of privacy is a moot point when you consider that students don’t really have the same rights as adults, especially in public school situations.