Just before half term, a parent walked into the playground of John White's Milton Keynes primary school and pinned a nine-year-old pupil to the fence.
The father, who claimed the boy had been bullying his own son, out of school, had decided to deal with it directly himself, without talking to teachers. The incident was reported to the police as an assault and they are investigating.
"That's never happened before. It was quite frightening," said Mr White, head of New Bradwell primary.
Heads at their annual conference this week claimed they and their staff are facing more attacks.
David Hart, NAHT general secretary, said: "We seem to have more people who believe that abuse or violene or threatened violence is the first and last resort."
He said, in the past year, at least 140 members had been subjected to physical assaults or threats of violence so serious they warranted parents being banned from school premises - double the previous year. Most cases (70 per cent) involved parental abuse, the rest was from pupils.
Since January 1999, the union has won pound;350,000 compensation for members, with possibly another pound;500,000 in the pipeline. Only last week, a nursery head was awarded pound;125,000 in criminal injuries compensation, after a 1995 assault by a parent resulting in neck, arm and shoulder injuries led to depression, sick leave and early retirement.
The Prime Minister Tony Blair this week promised that a re-elcted Labour government would bar disruptive parents from schools.