“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. […]

But while we are on the subject of adult failures, it should be noted that the Motion Picture Association of America’s ratings board, by insisting on an R rating for “Bully,” has made it harder for young audiences to see. The Weinstein Company, which is distributing the film, has released it without a rating after the association denied its appeal and after a widely publicized petition drive was unable to change the board’s mind.

There is a little swearing in the movie, and a lot of upsetting stuff, but while some of it may shock parents, very little of it is likely to surprise their school-age children. Whose sensitivity does the association suppose it is protecting? The answer is nobody’s: That organization, like the panicked educators in the film itself, holds fast to its rigid, myopic policies to preserve its own authority. The members of the ratings board perform a useful function, but this is not the first time they’ve politicianed us.