“Bully,” Lee Hirsch’s moving and troubling documentary about the misery some children inflict upon others, arrives at a moment when bullying, long tolerated as a fact of life, is being redefined as a social problem. “Just kids being kids” can no longer be an acceptable response to the kind of sustained physical and emotional abuse that damages the lives of young people whose only sin is appearing weak or weird to their peers.

And while the film focuses on the specific struggles of five families in four states, it is also about — and part of — the emergence of a movement. It documents a shift in consciousness of the kind that occurs when isolated, oppressed individuals discover that they are not alone and begin the difficult work of altering intolerable conditions widely regarded as normal.

The feeling of aloneness is one of the most painful consequences of bullying. It is also, in some ways, a cause of it, since it is almost always socially isolated children (the new kid, the fat kid, the gay kid, the strange kid) who are singled out for mistreatment. For some reason — for any number of reasons that hover unspoken around the edges of Mr. Hirsch’s inquiry — adults often fail to protect their vulnerable charges.

Alex, a 14-year-old in Sioux City, Iowa, whose daily routine includes being teased, humiliated and assaulted (especially on the school bus), cannot bear to tell his parents what is going on. He even sticks up for his tormenters, who he says are “just messing around” when they stab him with pencils and call him vile names.