A slight man ambled up to the microphone. “Hello, my name is Steve. I’m in here because I murdered my wife.” In the audience, 14-year-old Damian from Woodlands School in Gillingham bit his lip. His friend Danny stopped chewing a clandestine piece of gum. Clearly, this was going to be no ordinary school trip.

Today, the two boys are among 72 pupils who have swapped their well-lit classrooms for the Gothic gloom of Maidstone prison. They are there to take part in what might be described as an experiment in “aversion therapy”.

Can a measured dose of prison life vaccinate boys and girls with behavioural problems against slipping into a life of crime? Plenty of teachers, probation officers and police who work with juvenile offenders think it can. So too do the prisoners who run this unique programme at Maidstone.

Over the past four years more than l,000 schoolchildren and teenagers have passed through the forbidding portals of the prison. But unlike other visitors, they do get past the public areas where inmates can talk to family friends or legal advisers. They get inside the cells, walk in the exercise yard, examine the chains and cuffs used to restrain difficult prisoners and watch as their treasured possessions are taken away and bagged up for collection when they leave.

They also hear first hand evidence of what its like to spend your life inside. Chief Inspector Chris Cerroni of the Metropolitan Police explains: “Rather than trying to rehabilitate people when they have committed offences, we want to get to them at the beginning of the cycle - changing their values, and habits. ”

Alex is 16, Martin is 15 and Nick is 18. All three are familiar faces at Bexleyheath police station where they have been cautioned more than once for thieving, trespass or criminal damage. Today, they are in the station voluntarily with the police to set off on the Maidstone prison visit.

Nick knows a bit about prison life already, one of his mates has been inside. Martin and Alex are not sure what to expect. “We just wanna see what it’s like,” they shrug.

Matt is 13. He’s here because his mum caught him stealing from her purse and called the police. In despair at his persistent absconding from school and getting in trouble with teachers, she asked the police for him to be included on the trip.

In the van on the way down to Maidstone the lads talk loudly of cars and motorbikes. The mood is jocular, only Matt says nothing - but he scoffs a whole tube of Rowntrees pastilles - squeezing each one flat before he pops it in to his mouth.

Outside the prison gate 10 pupils from Woodlands School in Gillingham are also joking about the imminent visit. Andy, 15, reckons its better than being stuck in a classroom. Ben, l4, laughs, “I could handle it here. It’d be no worse than home.”

They are less happy when they have to submit to a full body search and hand over their sweets, cigarettes and penknives on their way through heavily padlocked doors, before emerging into a courtyard guarded by a mean looking Alsatian and his meaner looking handler.

First stop is the segregation unit for “difficult” inmates. While a prison warden explains the routine each group examines a cramped cell - which stinks of urine. Ben shivers. When he sees the “Box” - a solitary confinement cell not much bigger than a shower unit, his face pales.

From there to the chapel where a four-piece prison band provides a musical backdrop to a series of short talks from the inmates on the physical horrors and the mental anguish that is prison life. They heard “Big Ritchie” from Brixton explain how he swapped street life - fast cars, drugs and women for a crowded cell. “I thought I was one tough guy,” he said, “until I heard other men screaming in prison because they were being raped. I cried.”

Marcus told his rapt audience what it felt like to be cut off from your family and friends - with visits permitted just twice a month for half an hour. “Just think about it - that’s 12 hours a year. By the time I’m out I will have seen my wife and kids for six days.”

Two women prisoners from a nearby open prison speak to the girls in the audience about the humiliation of strip-searching. Finally each prisoner organises a makeshift “seminar” with a small group of children. For 40 minutes each child is told, in no uncertain terms, where they are heading if they don’t shape up at school.

“Most of the people in here can identify with these kids.” explains Steve. “They started off doing stuff like nicking cars - now they’re in for attempted murder.” He is angry that the media and some politicians portray prison life as a cushy number. “When The Sun says we are all drinking and having parties every night they aren’t doing these kids any favours. Prison is a shitty place. If we can stop just one of these kids from ending up like us in Maidstone it will be worth it.”

No research has been carried out on the effectiveness of the Maidstone shock therapy. As an unofficial programme, tolerated rather than promoted by the prison authorities, it has won no Home Office backing. But probation officers, police and teachers are enthusiastic about its impact. Angela Howe, of Woodlands School, says that “re-offending” rates of children who have been on the course are very low; she is busy trying to interest research bodies in the scheme to see if it can win more official support.

Peter West, probation officer for the lifers at Maidstone, has been a prime mover in getting the programme off the ground, encouraging the men to organise mailshots and liaise with the authorities. “I think it works because it’s led by the prisoners themselves. They don’t get any remission for doing this. They do it because they care about what happens to these kids and I think that message gets across.”

He cited a letter from the mother of one boy who wrote, “I don’t know what you have done - but it worked. My son was taking coke, getting drunk, dropping out of school. Now he’s stopped all that and has gone back to college and wants to make something of his life. Thanks.”

Outside, the visit over, the mood was sombre as the children returned to their minibuses . Jokes were thin on the ground. Ben admitted that there was no way he could handle prison life. Nick, Alex and Martin lit cigarettes and smoked all the way back to Bexleyheath. “No way I want to end up there,” said Alex to general nodding of heads. Young Matt, reaching for the last of his pastilles, said, “I never want to see that place again.” Let’s hope he never does.