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. 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." Media regulator Ofcom said protecting children was a ‘fundamental concern’.