A study by Edinburgh University examined two groups of children who started offending when they were very young.

They found that staying in school was the only major difference in whether they improved or deteriorated. The research, which will be published next year, is part of the Edinburgh Youth Transitions Study – a wider survey that has been tracking more than 4000 young people in Edinburgh since 1998.

The study, by Dr Susan McVie and Professor Lesley McAra, found those excluded at age 12 saw a huge increase in their predicted offending.

Professor McAra said: “It’s important to remember that many more youngsters are involved in offending than actually get convictions.

“Some 75% of boys said they were involved in offending but only 1% had convictions. The peak age of offending is 18, but we can show from individual trajectories that there are different patterns.

“The later onset group form the more normal pattern. The key factor which emerges is school exclusion. It is the key differentiating factor. It’s not rocket science.

The statistics we have are based on conviction probability from statistical patterns from the 10 years of data we have on criminal convictions.

“A lot of what the Government is trying to do – with the early interventions framework and the curriculum for excellence – is right, but fundamentally we need to get more imaginative at keeping kids in school and trying to help them in mainstream provision. We need to think about making the curriculum relevant to those not academically inclined.”

“It is vital not to stigmatise young people. Instead we need to saturate communities with known risk factors and support people through health and education rather than trying to pick criminals. We must not over-react to young offending.”

Figures released earlier this year showed that there were 39,717 exclusions in Scotland during the 2007-08 school term.

More than 4000 young people who started secondary school in 1998 have been tracked until the age of 19 through the Edinburgh study, by which time 355 – 9% – had at least one criminal conviction.

Of the 1140 children interviewed who left school at the earliest opportunity, 23% ended up with a conviction. Of the 489 excluded, 33% were convicted.

Ann Ballinger, General Secretary of the Scottish Secondary Teachers Association, said the emphasis needed to be placed on properly resourcing specialist units to help young people stay in education when the classroom was not appropriate.

“Some specialist units where young people get one-to-one or four-to-one support are seeing really good results, but we need to ensure that the appropriate money is spent on this because if you put these young people back into a classroom of 30 other children you will just alienate them,” she said.

“For a lot of them the reason for their behaviour is that they are completely disengaged because they have no understanding of what is being taught.

“I was struck by a study done in Polmont which found that 98% of the young offenders there struggled with basic literacy.”

Glasgow City Council has significantly reduced exclusions by intervening earlier with young people showing signs of struggling.

A YouGov poll earlier this year found that 75% of adults backed headteachers’ right to decide whether to expel or suspend unruly students, with just 18% in support of local authorities making the call.

The Scottish Conservative Party, which commissioned the research, said it demonstrated opposition to the present situation, where councils have ultimate control over exclusions.

The party’s education spokeswoman Liz Smith said authorities should bring in a system of “second chance centres”, where disruptive pupils could be educated separately from their peers.