The director of high-profile US documentary Bully has called on Mitt Romney to take a stand against bullying following revelations that the Republican presidential candidate made life a misery for a fellow pupil as a teenager.

Lee Hirsch, whose film examines the deaths of US schoolchildren Tyler Long and Ty Smalley, who killed themselves after being bullied, said Romney's failure to issue a genuine apology for his actions were a missed opportunity that he hoped the former Massachusetts governor would reflect on. "This could be a true presidential moment for Mitt Romney," he told the Hollywood Reporter. "My hope is that he would recognise that we are past framing bullying as horseplay or pranking around. We need our leaders to call it as it is. Part of that conversation is moving away from 'kids will be kids.'"

Hirsch added: "This is an opportunity for Romney to really lead on this issue. His apology fell short of that. That's not to say he won't feel different after some soul searching. I would hope anyone standing for president would take the opportunity to set a clear example on this issue. We're looking to do away with language that minimises bullying."

After being confronted with evidence via a Washington Post article that he led a gang which held down a student and cut his hair while a teenager at the private Cranbrook school in Michigan in 1965, Romney at first sought to deny any knowledge of the incident, before later passing it off as a "prank". He told Fox News Radio: "They talk about the fact that I played a lot of pranks in high school and they describe some that, well, you just say to yourself back in high school well I did some dumb things and if anybody was hurt by that or offended, obviously I apologise but overall high school years were a long time ago."

Hirsch's film, which follows students from schools in Texas, Mississippi, Georgia, Iowa and Oklahoma during the 2009-10 school year, was at the centre of a row between distributor the Weinstein Company and US censor the Motion Picture Association of America earlier this year after the latter initially handed it a prohibitive R rating due to instances of swearing. The organisation later backed down and issued a PG13 certificate after Harvey Weinstein threatened to release the documentary unrated in protest.

Hirsch estimates it has now been seen by 85,000 students and the film has even been parodied on South Park. "The school buses are pulling up to theatres around the country," said the film-maker, who was himself bullied as a teenager. "The film has been screened at the White House. I'm assuming president Obama has seen it."