The research by Warwick university for the National Union of Teachers shows that thousands of the capital’s teachers and children are at risk from pupils with knives, hammers and pellet guns.

One secondary teacher was shot in the arm while crossing the playground by a pupil using a pellet gun. Another school reported a child from a neighbouring school being stabbed outside its gate by a Year 10 pupil.

A fifth of the 49 mostly senior teachers responding to the survey said teachers and pupils had been threatened with knives and slightly fewer (18 per cent) said the knives had been used by pupils.

Fewer than two out of five schools routinely informed police of such incidents, despite official guidelines stating that they should. Schools also appear reluctant to take advantage of Department for Education and Skills rules permitting exclusion for a first offence involving weapons.

Only about a third permanently excluded such pupils.

Steve Sinnott, NUT general secretary, said: “This research shows how difficult it is for many teachers in inner-city schools. It also shows the commitment of heads and staff who do everything possible to avoid blighting children’s lives by involving the police.”

Incidents involving outsiders, such as parents or siblings, arriving in school with weapons were much rarer. Only one in 17 teachers said this had happened at their school although one reported a parent threatening a colleague with an iron bar.

Mr Sinnott said schools needed communities to have zero-tolerance of violence if future tragedies were to be avoided.

The findings come two years after 14-year-old Luke Walmsley was stabbed to death at Birkbeck school in Lincolnshire.

Most teachers in the survey did not believe school security had deteriorated during the past year. Only one in seven said their school had become less safe. Fewer than half of teachers support the use of metal detectors to search pupils, a proposal backed by the NASUWT, the second largest teaching union.

Almost three-quarters support the Government’s promise to allow heads to search pupils for weapons but fewer than half want that power delegated to other staff.

Teachers were also concerned that searching pupils could exacerbate problems by making them feel less safe at school. They preferred measures such as CCTV, and improved fences.

The study found two-thirds of schools had a formal policy for recording and dealing with incidents involving weapons. Of these, three-quarters routinely informed governors of incidents and slightly more than half told the police. Primary and nursery schools were more likely to adopt a laissez-faire approach with teachers happy to restrain young children themselves.