Some children are more likely to be criminalised for carrying a knife on the basis of their academic record or background rather than the risk they pose, education watchdog Ofsted finds.

A report, which looks at knife crime in London, reveals that schools’ decisions about whether to call the police to knife-carrying incidents can be based on irrelevant factors – such as a child’s prior attainment.

Ofsted warns that an inconsistent approach to police involvement risks “bias”.

“Children who have a certain demeanour, a particular type of relationship with their teacher or a type of background may likely be criminalised for the same actions that other children would not, depending on which school they go to, and even within the same school,” the new research says.

Amanda Spielman, chief inspector of Ofsted, has called for headteachers to be given more information and guidance on how to respond to knife-carrying and knife offences to provide clarity.

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The report – which looks at how more than 100 schools, colleges and pupil referral units in London are protecting children from knife violence – concludes that schools do not have the resources or ability to counter the complex societal problems behind the rise in knife crime.

It says the issues need to be tackled by a range of partners, such as the police, local authorities and policymakers, as a lack of partnerships is adding to inconsistencies in the way schools respond.

Ms Spielman said: “Some schools are valiantly trying to fund school-based early help services or other services that were once provided for free. But we cannot allow responsibility for this to be landed on schools in the absence of properly funded local services.”

She added that some heads are concerned they will be seen as a “problem school” that parents should avoid if they educate about knife crime, the risks of grooming and exploitation by gangs.

And some schools and colleges are not conducting knife searches to ensure they do not send the “wrong message” to families, the report says.

Ms Spielman added that there was a “harmful narrative” developing that exclusions must cause children to join gangs or carry knives.

But she said the issue was “too complex” to be reduced to binary arguments.

Her comments come after police chiefs linked rising knife crime to the exclusion and off-rolling of pupils.

The Ofsted boss said: “There is evidence that points to a correlation between the two, but of course this does not prove causation.

Owen Jones speaks about the UK's knife crime crisis on Question Time

“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.”

However, the Ofsted boss also warned that decisions to exclude pupils for bringing knives into London schools do not always take the best interests of that child into account.

Diane Abbott, Labour’s shadow home secretary, said: “The government’s policy of cuts has failed vulnerable children in particular and Ofsted has now said that this could be one of the factors driving the rise in knife crime.”

A government spokesperson 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.”

The report came as Sara Thornton, the outgoing chair of the National Police Chiefs’ Council, called for “concerted action at a national level” on school exclusions and other issues making children vulnerable to crime.

She told the Home Affairs Committee that police alone cannot solve the “many and complex” drivers of serious violence, following outrage over a wave of stabbings seeing teenagers murdered in the street.

Ms Thornton warned: “A lot of the organisations involved in prevention and diversion are just not there in the way they were five or 10 years ago.”

She called for additional funding to help officers control knife crime in the short term, including through increased stop and search and high-profile policing in known hotspots.

“We do not think that policing is the only solution, we think that when violence is getting to the level it is we need to immediately suppress it,” she added.

“There is a little bit of levelling off in London but outside virtually all the indicators are still going up.”

The senior officer warned that although the home secretary had publicly backed a “public health model” to prevent crime, the government had not given the funding or leadership to implement it.

Ms Thornton said the £970m funding increase for 2019-20, generated mainly through council tax, was “not enough” to increase capacity and that other areas of policing were suffering because of the response to violent crime.

Despite Theresa May’s claim that there was no direct link between the loss of 20,000 police officers across England and Wales and rising violent crime, Ms Thornton said: “There are fewer officers, there is a lot less policing happening and there is more crime … where we have so many young people dying in our streets we need a much more concerted response from government.”

Police chiefs have presented ministers with details of the resources they need for a “surge” in capacity to combat the rise in violent crime, following a meeting with Sajid Javid last week.

Speaking after the meeting, he said: “I think police resources are very important to deal with this. We’ve got to do everything we can.