Parents must take responsibility for what their children watch online, amid concerns that youngsters are accessing inappropriate material such as pornography, headteachers warned today.
They said misuse of technology is a “growing problem”, with schools forced to deal with the fall-out.
The National Association of Head Teachers (NAHT) also raised concerns that children as young as four are re-creating violent computer games in the playground because they do not realise they are make-believe.
Speaking as the NAHT meets for its annual conference in Harrogate, Kenny Frederick, a London headteacher, said: “What is a big issue in terms of mobile phones is parents buying their youngsters smartphones on which they can access pornography, anything - it's something we have no control over.
“Parents have to take control - trying to make sure parents understand the issues is a big thing.
“In terms of messaging and so on, we deal with the issues around that day in and day out. It's a growing problem. Schools have a part to play, but parents have to take control of that at home.”
Ms Frederick said pupils today conduct arguments through technology such as instant messaging, which cannot easily be taken back.
“We have to deal with the fall-out when there are allegations of bullying and so on down to the misuse of mobile phones and technology,” she said.
Sue Street, from Harrow High School in London, said headteachers need to reclaim social media and use it for positive purposes.
“One of the key things we must do is reclaim it for our own purposes, the school profession and contact with parents.”
Schools must set a “good example” on how to use social networking, Ms Street added.
The union also warned that headteachers are “increasingly aware” that young children are viewing inappropriate material.
John Killeen, a head from North Yorkshire who teaches in East Riding, said: “Headteachers are increasingly aware of children playing and being exposed to inappropriate software, films.”
These youngsters are replicating games that they have seen and that they are exposed to, he said.
“It could be combat, kung fu attacks, that they see people doing. They think they can replicate that in a play situation.”
Stephen Watkins, a head from Leeds, said: “Four-year-olds don't understand if you hit someone over the head with a brick they're not going to recover, they're not going to jump up immediately as they do on the screen.”
Delegates at the NAHT conference are expected to debate resolutions this weekend calling for the Department for Education to investigate the misuse of social media sites, and for the union's executive to work with Government to clarify child protection guidance to help schools dealing with situations where children have been exposed to inappropriate material.