Children are taking weapons into school and some pupils are wearing stab-proof vests to protect themselves from becoming victims of violence, according to the first-ever report on the impact of gangs on schools.

The report, given to The Independent, said teachers at one urban comprehensive where pupils are said to be "seriously involved in gangs" were "aware of young people wearing bullet-proof/stab-proof vests in school".

It cites one estimate that the number of pupils under 16 involved in gangs had doubled in the past five years.

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The report, commissioned by the NASUWT teachers' union and prepared by consultancy firm Perpetuity, is the first in-depth look at how youth gang culture is influencing schools.

It comes to the conclusion that children as young as nine at primary school are becoming involved with gangs – used as "runners" and "couriers" to ferry messages by older members.

"Some of the case study schools felt the problem had increased over the last few years with gangs becoming more dangerous involving children at a younger age," the research says.

"Some schools have problems with pupils carrying weapons in school. This can include young people who carry weapons and/or those who hide weapons in and around school grounds."

The most common weapons teachers reported seeing were BB air pistols and batons. In one incident a teacher saw a meat cleaver.

The report says: "Some schools have experienced the impact of gang culture and there are examples where former pupils have been killed as a result of involvement in gang-related activity. A secondary school in a gang-affected area may have 20 seriously gang-involved pupils, 40 less seriously involved and up to 100 marginally involved."

One pupil told researchers he was wearing body armour because of "needing to" – although attacks were more likely to take place on the way to and from school. Children had deliberately failed tests and exams "to remain being considered 'cool' by peers".

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The report suggests several measures to lessen the impact of gang involvement, such as sending children on prison visits to see the effect of loss of liberty, and staggering times to avoid clashes with other schools.

It cautions against automatic expulsion for pupils carrying weapons – warning that this could lead to them being more exposed to the influence of gangs on the streets, adding: "Excluding troublemakers from after-school activities could negatively impact on behaviour if they commit crime due to boredom."

Chris Keates, general secretary of the NASUWT, said: "Schools should feel free to exclude pupils if they are posing a threat to the education of other pupils. What we need to be sure of, though, is that if they are excluded on day one, on day two there is a permanent full-time place available for them – at some specialist unit – so they do not have to be on the streets."

The report says gang culture affects just a minority of urban-area schools.

'Weapons are brought into school – and used'

"It is only a matter of time in a school in the UK that something happens and a teacher is on the end of it." The speaker is a teacher at a troubled urban comprehensive which has been caught in the crossfire between rival gangs and is speaking about his fear that one day soon a teacher could be shot.

In the secondary school, which is in one of the most deprived areas of Britain, teachers regularly intervene to stop ex-pupils who are gang members coming on to the premises and causing trouble. Sometimes, the report says, the gang members drive their cars at speed around the playground "putting staff and pupils at risk".

The school serves a community where many local gang leaders are in prison. Pupils at the school are being used by gangs to transport firearms and drugs for them. "One member of staff talked in detail about under-16s being recruited by drug-dealing gangs to deal or run [transport] drugs. Others were used by older members to store weapons. The fact that local gang leaders were famous for cars, jewellery, women, etc, drove some young people towards the lifestyle."

Around 10 pupils at the school were "seriously" involved with gangs, up to 20 more associated with them. Weapons recovered on the school site include knives, BB guns and sharp instruments.

However, many pupils saw school as a "safe haven" compared with the outside – even though intruders had arrived intent to injure other pupils. "Staff in school had been injured by intruders," the report said. "Weapons had been brought on to site and used indiscriminately against members of staff trying to secure the school and protect individuals inside."

A police officer had been placed on site which had reduced violent behaviour for a while. However, a reduction in resources in that area had led to him being redeployed.