A fifth of England's secondary schools believe they have a problem with "gang culture", an Ofsted report suggests.
Ofsted has once again highlighted concerns about pupil behaviour
The education watchdog also said up to half of pupils in some schools showed signs of poor behaviour, mainly "low-level disruption".
It called for "strong leadership and effective teaching", after looking at 15 secondary and 63 primary schools and referral units for excluded children.
The government said it encouraged "zero tolerance" of disruption.
An Ofsted spokeswoman told BBC News that most problems with gangs were "perceived", with little evidence to substantiate the claims.
But, she added, it was important to address schools' concerns.
Ofsted's annual report, published earlier this month, showed that the proportion of "good or better" behaviour in secondary schools had declined from more than three-quarters to two-thirds since 1997.
Following the latest report, England's chief schools inspector, David Bell, said: "Although the large majority of schools are orderly places where children behave well, it is worrying that unsatisfactory behaviour has not reduced over time."
Weapon-carrying incidents were described as occurring never, or at the most once a year, in 60% of schools.
In just under 30% of the schools visited, there might be an incident each term and fewer than 10% estimated there were two or more incidents.
Ofsted said schools needed a strong sense of community, good links with parents and the use of information systems that tracked and analysed behaviour.
Well-maintained buildings had an influence on pupils' "pride".
The report recommends "regular training, focused on classroom practice, combined with an in-depth appreciation of child and adolescent development".
Amanda Haehner, a secondary school teacher in Croydon, south London, said: "There is a lot of talk about gang culture. It could be just a group of young people with their hoods up, who seem threatening, or it could involve more organisation.
"The nature of a school is that you get lots of people of the same age together, so the definition of a gang is difficult."
Ms Haehner, a national executive member of the NASUWT teachers' union, added: "We see incidents in schools where something kicks off and suddenly other people are summoned to the school gate.
"But it is difficult to say whether the people involved are in gangs or perhaps families or other groups."
Possible gang activity was visible through graffiti "tags" - motifs using made-up names for members - daubed on walls, she said.
Schools Minister Derek Twigg said: "We are supporting schools in showing zero tolerance to any bad behaviour.
"Permanent exclusions are 25% lower than 1997 and, as Ofsted acknowledges, pupil behaviour is good in most schools most of the time."
The institutions featured in the study - Managing Challenging Behaviour - were in Camden, Croydon, Durham, North Somerset, Shropshire, Stockport, Sandwell and Wiltshire.