A former murder investigator has called on schools to stop excluding pupils.

This could have a dramatic impact on violent crime and even help reduce homicide rates, said John Carnochan, who led a police initiative credited with a drastic fall in violent crime.

Mr Carnochan was a detective chief superintendent with Strathclyde Police until 2013 and, in 2005, co-founded the Violence Reduction Unit. Its approach to reducing violent crime - by seeing it primarily as a public health issue that must be dealt with pre-emptively - has in recent times been highlighted as a way of potentially tackling knife crime in England.

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“You can help [with] homicides,” he told an audience of secondary headteachers today.

“One of the major reasons” for reductions in homicides and in violence by gangs in places such as Glasgow, he said, was a dramatic fall in school exclusions.

“Because by keeping these young men in school, and doing something specifically for them, you not only stop them going on the street and getting lost but for the first time in their lives, somebody actually took time to sit down and say, ‘We’re going to help you solve your issues, we’re stepping alongside you’,” said Mr Carnochan, speaking at the annual conference of the Catholic Headteachers’ Association of Scotland.

Mr Carnochan, who was in the police for 39 years and was a senior investigating officer in murder inquiries, said society’s tendency to blame, shame and punish was less effective than attempting to nurture and understand people.

He told the heads’ conference - which has been meeting today and yesterday - that schools should be asking challenging pupils not “What’s wrong with you?”, but “What happened to you?”.

Schoolchildren who do not know if there will be any food or clean clothes when they go home, or if their dad will be sober, need teachers who will try to understand their behaviour and not merely offer glib advice, advised Mr Carnochan.

“Their world is completely chaotic, then you’re saying to them, ‘You need to stick in’?…They [just] want to get through the day,” he said.

A teacher might provide the consistency in a pupil’s life, and school might be one of the only safe places they know, he added.

Mr Carnochan, who has previously argued for a large reallocation of funding from higher education to preschool education, also said that school starting ages in the UK are too young - at four or five, children start school earlier than in most countries.

“We send kids to primary school two years too early, all the time - they shouldn’t be going to school until they’re seven,” he said. “They should be out there playing, falling out of trees and skinning their knees.”