The National Association of Head Teachers, which carried out the survey, says that the courts and the police needed to ensure the safety of heads and to take the problem seriously.

We need a deterrent here. Everybody working in a school is entitled to protection from violence.

David Hart, National Association of Head Teachers

"The sad fact of life is that incidents of violence and levels of abusive behaviour from parents are on the increase," said the NAHT's general secretary, David Hart.

"We need a deterrent here. Everybody working in a school is entitled to protection from violence ... We need to make sure that the police and the Crown Prosecution Service prosecute these people. And we need to bring home to magistrates that these offences need to be treated with the maximum possible severity."

As an example of how the problem could be downplayed by the police, James Bishop, head at St Mary Redcliffe primary school in Bristol said: "If colleagues telephone the police over issues to do with verbal or even physical assault, the police don't even come. They say they haven't the time, or that it isn't a top priority."

As well as fears over violence, head teachers also identified too much paperwork, inspections and bidding for grants as obstacles to their work.