By Katherine Sellgren BBC News Online at the NAHT conference in Torquay

Poor behaviour in schools is driving middle class parents to take their children out of the state sector, head teachers warned.

And aggressive and abusive parents are often more of a problem for teachers than their offspring.

Who amongst us cannot relate the growing incidence of nasty and vindictive behaviour by parents?

Monica Gatt, head teacher

Delegates gathered at the annual conference of the National Association of Head Teachers said a zero tolerance approach must be adopted towards parents who threatened teachers and heads.

Head teacher Monica Gatt said the media and the public had finally woken up to the problem.

"A significant minority of parents have such bad attitudes to schools and to the education system as a whole that, either by accident or by design, they are failing their children," she said.

"Who amongst us cannot relate the growing incidence of nasty and vindictive behaviour by parents?"

School staff were all too often subjected to appalling tirades of personal abuse, almost always in public areas, she said.

Middle class exodus

NAHT general secretary David Hart said bullying parents were a bigger threat to discipline and harmony than "thuggish pupils".

"Pupil and parental behaviour deteriorates year on year," Mr Hart told the conference in his annual address.

Mr Hart says bad behaviour is driving the middle classes away

And nine out of 10 teachers said bullying parents were more of a problem than their children.

When pupils were excluded, they were too often protected by parents and support groups who were turning "exclusion appeal hearings into quasi-judicial confrontations that would do credit to the Old Bailey", he warned.

"There is a danger that bad behaviour will increase the number of middle class parents heading for the private sector.

"If people believe that bad behaviour is having an impact on the rest of the school and on standards, they will - if they can afford it - vote with their feet," said Mr Hart.

"This is just the opposite of what we want to see, because we want to demonstrate is that the state sector is capable of providing for the most able children in the country.

"The fight for the middle classes hangs in the balance," he said.

Cash injection

In his speech, Mr Hart also called for a �15bn injection into the state education sector over the next five years.

Mr Hart's words came in the run-up to the government's comprehensive spending review next month, forming a clear message to the Chancellor and the Secretary of State for Education that the needs of the education world must be met.

"Ministers cannot ignore the suspicion in schools that education has dropped down the pecking order," he said.

He also raised concerns about teachers' and head teachers' excessive workload and the problems many head teachers experienced in recruiting and retaining teachers.