Children 'see harmful TV material', says National Association of Head Teachers
Children are still being exposed to "harmful material" on television despite strict rules on what can be shown before 9pm, say head teachers.
The National Association of Head Teachers - which has drawn up a charter to protect children's innocence - says the media must ensure pre-watershed content is free from sex and violence.
A poll of 1,013 parents found 96% believed the rules were being broken.
Media regulator Ofcom said protecting children was a "fundamental concern".
Speaking at the NAHT's conference in Birmingham, its president Gail Larkin said: "We know that most parents are trying their best to protect their children from certain dangers in the outside world.
"I know, as a grandparent, how much more difficult it is with such advanced technology to prevent children from accessing, often unwittingly, materials and media that are not just inappropriate but often obscene and mentally damaging."
The poll of parents in England, Wales and Northern Ireland for the association suggested that almost all of them worry about their children accessing inappropriate material.
Just under a third had installed parental controls on digital devices which stream content from the internet or access television programmes.
Lancashire head teacher, Amanda Hulme, who will propose a motion that the union adopt a new charter on the issue, said schools and parents shared concerns about the problem.
She cited instances of popular television soaps using adult storylines and examples of swearwords being used on television shows before the 9pm cut-off point.
She also raised concerns about the content of some music videos, which children may see on digital devices, and the impact of violent games.
An Ofcom spokesman said the protection of under-18s from inappropriate material was a fundamental concern.
"There are clear broadcasting rules designed to protect children, which we actively enforce.
"We recently issued new guidance on the TV watershed, warning broadcasters to be more careful about programmes they show before 9pm that could be unsuitable for children."
"We constantly monitor audience attitudes and our research shows that the vast majority of adults believe the current level of TV regulation is about right."