Lee Hirsch remembers being bullied as a child but it was not until he made a moving documentary on the subject that he recalled one disturbing incident.
''I remember being cajoled into a bathroom where kids had steamed it up and were acting out Nazi scenarios as if it was like a gas chamber,'' he says. ''I was attacked inside there.''
Other violent incidents that forced Hirsch to change schools inspired him to make Bully, a raw and powerful look at the lives of five American students dealing - or not dealing - with similar experiences.
Even at a Sydney screening for critics there were tears over the struggles of Alex, a lonely 12-year-old who is subjected to daily taunting and violence; Kelby, a sparky 16-year-old who has come out as a lesbian; Ja'Meya, a 14-year-old who is locked up after fighting back by taking a loaded gun onto her school bus; and the shocked families of two boys who have taken their own lives.
A US Department of Education study suggests they are among more than 13 million American children bullied every year, with nearly 200,000 avoiding school out of fear of what might happen to them.
''People really disagree as to whether it's serious, what it is, whether it's normal, whether it's a rite of passage,'' Hirsch says. ''So I thought if I could document [it] … if I could let our audience walk in the shoes of kids that are going through bullying, that it would be really helpful.''
The documentary shows bullying's many forms and chronicles the emergence of an anti-bullying movement through such organisations as Stand for the Silent.
Hirsch spent a year filming at a school in Iowa, where he discovered Alex's trip to school was a daily ordeal. What's surprising is how much he was able to capture on camera in a school whose senior officials believed they had the issue under control.
''A lot of documentary filmmakers will tell you that in time people stop noticing you,'' Hirsch says. ''Kids were responsive to the camera for the first two weeks, then it very quickly became a non-event that I was there.
''I was very low profile - I shot with a stills camera - so it never looked like a movie was being made. And in that world, they were used to bullying Alex and it hadn't really been stopped.''
Bully became controversial on its American release when the classification authority gave it a rating that stopped under-17s seeing it without a parent or guardian. Distributor Harvey Weinstein initially released Bully without a rating before it was reduced to the same PG-13 as The Hunger Games. In Australia, it has been rated M with the consumer advice ''mature themes and coarse language''.
''I think it's unfortunate that violence is something that's not a grave concern to censors but language is,'' Hirsch says. ''It was awesome to see the power of citizen advocacy. There was a petition by a 17-year-old girl that got half a million signatures.''
Rather than just strong, angry and troubled children intimidating the vulnerable, Hirsch sees bullying as a more complex social issue.
''Some people would say that these kids that bully are really on their way to a life of crime, or they're just little sadists in the making,'' he says. ''That's no less likely than they've just figured out how to game the system that they're in - that cruelty and intimidation is a path to social climbing. It has so much to do with the ecosystem of their school, that building, their world.''
Hirsch believes the film has changed how the issue is being dealt with in the US. ''People are moving away from crisis to a sense of action,'' he says. ''I see things happening in small towns, in town halls, in school districts, in state legislatures.
''We've screened at the White House, we've screened for Congress. We've seen legislation get backed after screenings, we've seen very, very entrenched conservative thinkers get behind this movie … and that's amazing.''
Bully is out on Thursday.