Kids who feel ridiculed are mad as hell and they're going to kill. The world is full of cliques and schools are the perfect microcosms of that. Geeks, sportos, brains, everyone who has been to school can picture these kids. We were either one of them or part of some other group. And just because we get older, it doesn't mean the cliques go away - they just have different labels.
Secretaries talk about bosses, bosses talk about the corporates and workers talk about the suits.
It may not be as emboldened as it is in high school, nor as segregated, but it exits. We're just more civilized when we get older. We talk about people behind their backs. The fact that children pick on each other is ageless. That the ones being picked on are armed is tragic.