The revelation came as the biggest study of children's access to firearms was launched and a guns amnesty in Welsh schools yielded 50 airguns.
Detective chief superintendent John Coles, said: "We have seen a development, not just in black communities, of a fashion culture for some kids to be carrying a gun. It is seen as cool."
The Met has commissioned a survey of 20,000 secondary pupils in the six London boroughs covered by Trident, to gauge the level of crime among 11 to 16-year-olds. Youths in Hackney, Haringey, Lambeth, Brent, Southwark and Newham are being questioned about weapon-carrying, drugs and teenage pregnancy, by the charity Communities That Care.
Teacher unions predict it will confirm their fears about the level of weapon use among school children. In a survey last year of 160 secondary teachers almost one in 10 claimed to have caught pupils with guns in classrooms.
In Wales, 50 airguns were handed into police stations last month following a two-week campaign aimed at pupils. Pc David Roome, who is running the north Wales campaign, said: "Teenagers think airguns are toys but they can cause real injury. They should not be in possession of them."
The TES has spoken to one north London school where three children, one aged 13, have brought in realistic-looking ball-bearing guns. One 16-year-old sparked a security alert by brandishing one in the school grounds.
And in County Durham a 12-year-old boy was permanently excluded in November for pointing a gun at teachers. John Brierley, the head of Blackfyne community school, said: "He apparently borrowed the gun, took it to the school and threatened a teacher with it.
"It was not loaded but the threat was just as real. He even appeared to have told others he was going to do it beforehand."
In Walthamstow, north-east London, a 13-year-old died when a friend put a pistol to his head as a "prank" and pulled the trigger. Renelle Coke, 18, was jailed for two years last week following the killing of Dean Davis in July 2003.
Chris Keates, acting general secretary of the National Association of Schoolmasters Union of Women Teachers, said: "There have been a number of instances in the past five years of BB guns being brought in and fired."
Her union has called for airport-style security to be installed at school gates. This week one company based in Oxfordshire told The TES it had sold a security metal detector to a private school in the past year. Another firm also confirmed it had school clients but refused to say where.
Mr Coles said: "There are some parts of Britain where the culture has generated extremes of violence where the staff and other pupils need some sort of protection. But I think few need the security measures being talked about here."