[seles]

I would guess that what she was doing was causing the bottle to pop by doing a chemical reaction inside it that creates lots of gas as a by product of the chemical reaction, enough to cause a significant increase in pressure until the bottle pops.

This is actually something that kids did when I was in high school 10+ years ago (although never at school or during school hours). I don't think it is a good thing for kids to be doing, but I do think authorities are over reacting (pun intended), it is not a real explosion or bomb, just a bottle popping. However I do think someone could get hurt if they were right next to it when it pops.