The 20-year minimum sentence handed out to the killer of teacher Ann Maguire is too harsh and puts Britain out of step with more lenient attitudes in the rest of Europe, according to a leading youth justice campaigner.

Will Cornick, who was 15 when he stabbed to death the Spanish teacher in April, was told that he may never be released when he was sentenced to at least 20 years on Monday by Mr Justice Coulson.

Announcing the sentence, the judge said that he was going for a higher tariff than the minimum of 12 years for the crime because of seven aggravating factors. These included the fact that it was premeditated, committed in public, in front of children and witnesses who were traumatised, and that Maguire had died in “extreme pain”. Leeds crown court heard he hoped to kill two other teachers, including one who was pregnant. He told doctors his plan was to stab her in the stomach to kill her unborn child.

That penalty is the longest given to a child in Britain for more than a decade, according to the Standing Committee for Youth Justice.

Its chairman, Penelope Gibbs, said Cornick should have been given a chance at rehabilitation by being sent to a specialist psychiatric jail such as Grendon in Buckinghamshire, which tries to reform violent criminals through group therapy.

Speaking on BBC Radio 4’s Today programme, Gibbs said: “Everybody has the capacity to change. It seems clear that he [Cornick] had serious mental health issues. He should not be released until he has changed sufficiently to make society safer. But there is no evidence that will take 20 years. There are very good prisons, there’s a prison called Grendon, which is a therapeutic community, and prisoners go there for a few years, and it has great results.”

Gibbs, a former magistrate, claimed nowhere else in western Europe would impose such a long sentence on a child.

She said: “We are out of line with the whole of western Europe. There are no other countries in western Europe which give children, and this boy is seen as a child under the UN convention on human rights … a life sentence.

“We think this is the longest sentence given to a child in at least 10 years. I’m not going to tell you what the right sentence would have been, but 20 years and a life sentence is too long.”

Gibbs asked: “What do we want from the justice system? What I want is that those who offend come out less likely to offend. Punishment is also incredibly important, particularly for the victims and families, but the fact is how many years do we need for punishment?

“We have given him a sentence which is more than his own lifetime. He was 15 when he did this crime and we would say you don’t need that long to punish.”