Girls are often drawn in by the promise of friendship but can end up drinking, using drugs and carrying out muggings, according to School-Home Support, a charity that works with schools to tackle the problems.
Most of the girls involved are aged 13 upwards, but some Year 6 pupils are getting caught up in gang activity, especially when they have older siblings.
The findings follow a government warning earlier this year that gangs were targeting primary children to act as drug runners.
Guidance for schools on how to combat the issue has been promised by ministers, but it is still being prepared by the Youth Justice Board.
School-Home Support has 170 staff based in schools in 14 London boroughs. Its workers offer support to pupils and parents in reducing exclusions and improve attendance and behaviour.
Jenny Deeks, a regional director with the charity, said: "Girls joining gangs is an increasing phenomenon. They get drawn in because they think it will give them somewhere to belong, but things regularly turn abusive.
"There are often initiation ceremonies which involve shoplifting. There has been a notable increase in the number of muggings carried out by girl gangs. It's frightening."
Teenage girls are less likely than boys to carry weapons but there are still regular incidents of violence, according to School-Home Support workers. Older gang members target younger girls who are then used to lure other girls to areas where they are mugged.
Teachers are often unaware of the extent of the problem. Ms Deeks said: "The gang activity is often very separate from school with girls coming from a number of different schools. Teachers need to be aware that there is an issue in the inner-city and it will be having an impact on children's education." Ms Deeks urged schools to offer training to staff to deal with gang issues.
Graham Robb, the interim chair of the Youth Justice Board, wants to start a scheme where schools are given special recognition if they undertake work to eradicate gang problems. But he admits many schools steer clear of tackling gang-related issues for fear it will get them a bad reputation.