Seven minutes of film, stills from which were published in a local newspaper, showed the Year 8 boy holding his head after being verbally and physically abused.
Tim Davies, headteacher of Addington high in Croydon, south London, said the attack was deliberately orchestrated for the camera.
Lord Adonis, the schools minister, was visiting Addington high on the day the local paper broke the story, although Mr Davies did not know whether the minister had seen the pictures.
Mr Davies watched the video clip at the offices of The Croydon Advertiser, after the older sister of the girl who took the pictures decided to hand them to the paper. "I was very upset and disappointed," he said. "The behaviour of some of those involved is usually exemplary. They are good kids who have let themselves down. They've welcomed the chance to unburden themselves of the guilt they felt about it." Mr Davies declined to say whether any children had been excluded.
An Ofsted report praised the school last year for the way it handled bullying. "Behaviour is good and the overwhelming majority of pupils feel confident because bullying and racism are dealt with in a firm and consistent way," it said. "One said 'I used to get bullied at my old school, but they sort it out quickly here.'"
Guidance published last month by the Department for Education and Skills made it clear that teachers are legally allowed to confiscate mobile phones if they are being used to harass other children.