Half of the heads questioned had seen knives in schools

The Guardian, which spoke to 829 heads, found 50% of those working in secondary schools had seen a pupil carrying a knife during the past year.

The National Association of Head Teachers said fully trained police, not school staff, should carry out checks.

Ministers are considering allowing knife searches of entire classes.

Union backing

The Guardian survey found 17% of primary school heads had encountered a child carrying a weapon during the last year.

Two-thirds of secondary heads and 58% of primary heads wanted police to enter schools.

Education Secretary Alan Johnson is said to back changing the Violent Crime Reduction Bill, currently going through Parliament, to allow whole-class knife searches by teachers.

But the National Association of Head Teachers said it should be police officers carrying out checks.

Its general secretary, Mick Brookes, said: "I think the feeling is stronger than the survey suggests in favour of police doing searches.

"In no way should teachers be involved in body searches of young people who are in possession of knives or other weapons."

'Sinister'

He added: "There are three groups of knife-carriers. There's one which carries them because they think it looks cool.

"The second has knives so they can defend themselves. Both of these are extremely dangerous.

"Then there's another group which is more sinister: the pupils who carry weapons with intent.

"There's no way someone untrained or unused to dealing with weapons-carriers should have to deal with any of them."

According to the survey, a third of secondary heads thought classroom behaviour was improving - but 42% felt it was deteriorating.