I remember back in boarding school, for a while whenever we would turn on our belly, in a fraction of a second we would have a human pile stacked one on top of another on our back. It was meant as a joke though, not harassment or anything like that. The one who caught you off guard suddenly jumped on you, lying flat on top and pinning you down, and called out for others to join in.
Yeah those were the days. Anyway, is there a word for this? I'm guessing if there is, it must be an informal one like wedgie.