A call comes through to DCI Steve Reardon. A 14-year-old boy has been stabbed in Liverpool. It’s serious, but he will survive. At this stage, there is no known suspect and the victim is refusing to cooperate.

DCI Reardon, a seasoned detective in Merseyside police’s serious and organised crime unit, spurs his team into action.

This is how most investigations begin. Last week the task of solving the crime fell to 15 teenagers, who were given the role of trainee detectives in a groundbreaking initiative aimed at steering vulnerable young people away from a life of crime.

The youngsters, most aged 14 and from an area of Liverpool blighted by gangs, drugs and knives, began the week collecting their warrant cards before landing their first case. Over the course of a week, they interviewed the victim, his family and the chief suspect – all played by actors – and gathered enough evidence to take their investigation to court.

Some of the boys were at first visibly uncomfortable around the police – or “bizzies”, as they called them – and sat with hunched shoulders, their coats zipped despite the stifling temperatures. But as the week progressed, the children relaxed.

They managed to coax crucial intelligence from the victim, who reluctantly told them he had been “moving packages” for his associates when he was knifed. The next day, they picked out their chief suspect, Mick, in a Viper parade – police code for a digital ID parade.

In interview, Mick denied being involved in the stabbing, insisting that he wasn’t in the area or driving a BMW that was spotted by the trainee detectives. His denials did not hold up in court, however.

Under cross-examination by the barrister Ken Grant, 16-year-old Mick Keegan admitted lying to police before letting slip his dramatic confession, telling Liverpool crown court: “I done it because he was doing my head in – I didn’t mean for him to go to hospital!”

The trainee detectives smiled self-consciously as their defendant was sentenced to three years in prison by the judge, Neil Flewitt QC, who praised them for their rigorous investigative work. Outside court, Harry, a 14-year-old nominated as the senior investigative officer, told the waiting media (the Guardian) he was very satisfied to put “Mick” behind bars. “It is a good result, getting dangerous people off the street.”

Pupils from schools across Merseyside taking part in a week-long activity at Merseyside police’s training academy in St Helens. Photograph: Colin McPherson/The Guardian

The initiative, run by Merseyside police and Everton in the Community, comes as police forces across the country grapple with ways to stem the soaring rise in knife crime across the country. Fatal stabbings are at their highest levels since records began in England and Wales, and the number of knife and offensive weapons offences has risen to its highest in nearly a decade.

On Merseyside, knife crime has more than doubled since 2011, to 1,231 cases recorded by police in 2018 – a 35% jump on the previous year. The increase has not gone unnoticed by young people. “Around the corner from mine, it was all sectioned off a few weeks ago,” said Keira, 14, one of the youngsters involved in the trainee detective scheme. “It makes you realise how close it can be and you’re not as safe as you should be.”

Harry said he had heard of kids his age carrying knives, but would not do it himself. The programme had made him realise the difficulties faced by police in solving violent crime, he said: “It’s harder for the police to get information than it is for a normal person.”

Some of the youngsters involved in the programme have been identified as at risk of slipping into organised crime, such as county lines drugs gangs. The aim of the scheme, police say, is not to recruit the next generation of detectives, but to prevent them from falling into violence. They want these 15 youngsters to warn their schoolfriends about the brutal realities of gangs.

“This is completely different to what my predecessors would ever have done,” said DCI Reardon. “The days of a police officer standing in front of young people wagging a finger are over.”

Earlier this month a 12-year-old boy was arrested for stabbing a married gay couple, in what police described as a homophobic attack, as they walked home from a pub in the Anfield area of Liverpool. A week earlier a BBC report suggested that teenagers in the city were being offered £1,000 by gang leaders to stab other youngsters – an incident DCI Reardon insists was isolated and not part of a wider trend. On Sunday a young man, reported to be aged 21, was stabbed to death in broad daylight outside a shopping parade in the Kirkby area of the city.

Rob Jackson, an emergency nurse clinician at the Royal Liverpool University hospital, showed the children a gruesome selection of images of some of the stabbings he had dealt with over his 20-year career: the severed hand of a man who owed £70 for drugs; a man with a £50 cannabis debt who was given a “Chelsea smile” – his cheeks sliced from the corners of his mouth.

The children learned about the principles of investigation and the difficulties of obtaining evidence. Photograph: Colin McPherson/The Guardian

There was another disturbing trend, he revealed, known as getting “bagged” – where victims are knifed deliberately in the bowel area so that they have to wear a colostomy bag.

The chances of surviving a knife attack are purely down to luck, said Jackson, whether from a single stab wound or several. The brutal reality is underlined by the fact that the last eight under-18s fatally stabbed on Merseyside were killed by a single blow.

Sam Cook was celebrating his 21st birthday when he was murdered with a single stab wound to the heart on a night out in Liverpool on 21 October 2017. His killer, who was on licence for stabbing another man with a machete, was jailed for life and ordered to serve a minimum of 28 years in prison last June.

Cook’s parents, Gill Radcliffe and Alan Cook, should be watching Sam get engaged, married, go on holidays and have children. But last Friday, 20 months after his death, they interred his ashes beneath a headstone bearing his name. “We’ll never see him again and that’s that harsh reality of knife crime,” said Radcliffe in an emotional meeting with the youngsters in Liverpool last week.

The detective leading the Merseyside programme is DC Dominique Walker, whose brother Anthony was killed in one of the most high-profile race murders in Britain 14 years ago. Barely two months ago, Walker’s childhood friend, Bala Lloyd-Evans, was stabbed to death in Liverpool city centre on the night of the Champions League final.

“The country is in a crisis for young people as we speak,” she said, adding that a string of “social harms” including austerity, cuts to youth programmes and cuts to education grants were driving children to violence. “This programme is a small cog in a number of different initiatives that should be taking place around the city and around the country,” she said. “It’s to save lives.”