Yeah I've seen that demonstration in school too. But if the teacher was willing to do it in school, with kids, how dangerous was it really?

Along with the other commenter, I'll add that a classroom is usually a lot bigger than a home dining room or other domestic party locations. That size also helps things dissipate instead of reflect. Not sure by how much but I'm sure it does something.

You can get permanent hearing damage from that demonstration if you stand right next to that balloon.