These students were a victim of school board cheapskates. You may be shocked to learn these stages are actually hollow. The one in my school actually had huge drawers on rollers within the cavity. Only a few wooden beams were holding the stage up. There was also a mattress and some beer cans under there.
If they had made the stage out of a solid wood block, no one would have been hurt.
Why is this event even getting all this press and news conferences like the building was bombed?? What did you think was going to happen when you put 30 people on a high school stage and they are all jumping up and down?