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.
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.
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.