Heads warning over smart phone pornography
Parents must take greater responsibility for the material their children are accessing on computers and smart phones urge head teachers.
Some heads say pupils are viewing pornography and other unsuitable material on phones bought by parents.
Others say they increasingly see pupils as young as four re-enacting violent computer games in the playground.
The National Association of Head Teachers says parents need to be aware of the dangers of technology.
Speaking as the NAHT annual conference got underway in Harrogate, head teachers said they wanted more guidance from government on how to deal with pupils who have access to inappropriate material.
Kenny Frederick, a primary head teacher from London, 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.
"It doesn't start in school, but it ends up in school and we have to deal with the fallout when there are allegations of bullying and so on down to the misuse of mobile phones and technology."
John Killeen, a primary head teacher from North Yorkshire, said head teachers were increasingly aware of children having access to inappropriate software and films.
"They're replicating the games. It could be combat fighting, kung foo attacks. They see people doing it and they think they can replicate that in play situations."
'Reclaim social media'
Stephen Watkins, a primary head from Leeds, said young children were often not able to distinguish between fantasy and reality.
"Four-year-olds don't understand that if you hit someone over the head with a brick they're not going to recover immediately and jump up - as they do on screen."
But Sue Street, a school leader in Harrow, London, said it was important for schools to embrace modern technology.
"We're encouraging heads to engage with social media," she said.
"Using it and setting a good example is one of the best ways of stopping children and parents from engaging in some of the other elements of it.
"We've got to reclaim it [social media]... we're hoping more and more heads will get involved."
The NAHT conference will debate social media over the weekend and is likely to call for more child protection guidance from government.