Knife crime and school exclusions are likely to be two symptoms of the same underlying problems, according to Ofsted.
The chief of the inspections body, Amanda Spielman, said there was a “harmful narrative” developing that exclusions must cause children to join gangs or carry knives.
But she added the issue was “too complex” to be reduced to binary arguments, and that schools cannot tackle the issue alone.
Ms Spielman said: “Schools can and should play their part, and many are.
“But this has to be as part of a broader coalition, with the support of local partners and the police.”
In response to an Ofsted report on safeguarding children in London from knife crime, the chief inspector warned that decisions to exclude pupils for bringing knives into London schools do not always take the best interests of that child into account.
She highlighted that some schools are not conducting knife searches, or teaching about knife crime, because they worry about how it will make them appear.
Ms Spielmam said: “Many school and college leaders we spoke to were trying to educate children about the dangers of knife crime and the risks of grooming and exploitation by gangs.
“However, some are concerned that if they do this they will be seen as a ‘problem school’, and subsequently avoided by parents.”
The report published on Tuesday states dialogue is missing between local safeguarding partners and schools about the purpose of searching, the impact on staff and pupils and evidence of the impact on knife-carrying.
It says: “While some schools told us that they had been offered wands, for example, they did not use them because the wands can only detect metal – as opposed to drugs or other banned items or substances that pupils might bring to school.
“Additionally, some schools were wary of beginning to search children in case it sent the wrong message to parents – that suddenly their children were less safe – or because the school 100 yards away did not.
“This was particularly a concern for colleges, which felt that it would make them look less safe than competing schools in their area.”
The research is based on responses from more than 100 secondary schools, colleges and pupil referral units across the capital.
It comes after police chiefs linked rising knife crime to the exclusion and off-rolling of pupils.
Ms Spielman said: “There is evidence that points to a correlation between the two, but of course this does not prove causation.
“It seems just as likely that exclusions and knife crime are two symptoms of the same underlying problems, exacerbated by cuts to local authority children’s services.”
The report says schools take an inconsistent approach to dealing with pupils who bring blades into schools.
While Ms Spielman raised concerns about the way some exclusions are carried out.
She said: “What we found through our research is that exclusion decisions in cases of children bringing a bladed object into school do not always sufficiently take into account the best interests of the child, which have to be balanced against the wider needs of the school community.
“Similarly, we found that schools’ decisions about whether or not to involve the police in an incident can be based on a variety of factors, not always relevant.
“It seems sensible to reflect on whether the child has any known connection to adults with a criminal history, but it is much less relevant to consider, as some schools told us they did, the child’s academic record.
“Headteachers clearly need more in the way of information and guidance.”
The chief inspector of schools said permanent exclusions have risen in the last few years, and there is a shortage of registered provision for excluded children.
She added that schools and local authorities need to work together to improve education and other preventative work, to reduce the need for exclusions.
“Exclusions are a necessary and important sanction, but it is not acceptable, or legal, to exclude without due regard for the impact on and risks to the child being excluded,” said Ms Spielman.
A Government spokesman said: “The issues surrounding knife crime, gang culture and bad behaviour at school are complicated. We should not draw a simple causal link from exclusions to crime – as Ofsted themselves argue.
“Any form of violence in a school is completely unacceptable and we have strengthened teachers’ powers so they can take action, including searching pupils, if they suspect a pupil has brought a prohibited item – such as a knife – into school.
“No headteacher ever takes the decision to exclude a pupil lightly – but it is right that a school has the ability to permanently exclude when that last resort is needed.”
Shadow home secretary Diane Abbott said: “This report is very timely and highlights the importance of early interventions in our schools and colleges to help protect children.
“The Government’s policy of cuts has failed vulnerable children in particular and Ofsted have now said that this could be one of the factors driving the rise in knife crime.
“Labour is committed to a public health approach to knife crime, which brings together schools and other public services along with increased police numbers to tackle knife crime from every angle.”