School staff should trawl social networking sites such as Facebook and Bebo to identify whether their pupils are gang members, according to advice in a government report.

Children, even those from primary schools, are being sucked into violent gangs, says the report, which gives guidance on how teachers can identify those at risk.

Other indicators of gang membership include the wearing of a bandana, or certain items of clothing or jewellery, particularly of one colour. Some pupils have even worn clothing that protects against weapons, it said.

Teaching unions welcomed the guidance, saying that some schools faced a growing problem of weapons being brought into classrooms.

Schools should also be alert to signs of gang activity, including graffiti “tags” on buildings or schoolbooks, particularly those referring to postcodes or neighbourhood street names.

The advice referred to a recent report, which said that some schools were located within a gang’s turf, and as such were at risk of becoming a “symbol of the territory and a site of contention between rival gangs, or a fruitful recruitment site for new members”.

The guidance said that teachers should listen out for threats of violence and extortion for money or goods, or robbery, and be aware if pupils are playing truant at the same time as other members.

It recommended being aware of the “use of extremist language or materials, sudden changes in friendship groups, sudden acquisition of expensive possessions such as designer clothes and top-of-the-range mobile phones and trainers, and carrying weapons, including replicas”.

Such weapons were carried for self-protection and also for reasons of fashion. Schools should be conscious of pupils showing “overly sexualised behaviour”. “More often girls are subservient in the male gangs and even submissive, sometimes being used to carry weapons or drugs, sometimes using their sexuality as a passport or being sexually exploited in initiation rituals in revenge by rival gangs.”

Primary schoolchildren, known as “tinies”, work their way up through symbolic acts of crime, the advice said.

Christine Blower, acting general secretary of the National Union of Teachers, said: “A minority of schools face increasing difficulties from weapons brought on to school premises. These schools need all the support they can get. If necessary, specific funding should be available to local schools for such additional support.”

John Dunford, general secretary of the Association of School and College Leaders, said: “This guidance is a proactive measure. It is meant to help tackle problems before they start. The reality is that schools are an oasis of morality in a world that, for many young people, does not give guidance and role models of good citizenship.”

Teachers could talk about gangs during lessons but should be careful not to glamorise the subject, the guidance said. Schools should also consider screening for weapons, using airport-style scanners or random searches.

Beverley Hughes, the Children’s Minister, said the guidance was issued because “schools are uniquely placed to spot the early signs of pupil involvement in gangs, and to work collaboratively with other agencies to tackle it”.

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