They say crime doesn’t pay, but Seb earns pound;9 a week in his prison job. Nearly enough to buy the CD he stole that started his journey from schoolboy to jailbird.

After slipping the CD under his coat and eluding the store detective, Seb thought he had got away with it. But the police turned up at his house later, gave him a formal reprimand and told him that if he broke the law again, he’d be in bigger trouble. Soon afterwards, he was caught spray-painting a school wall. Then his girlfriend left him and he went off the rails. High on drink and drugs, he attacked the owner of an off-licence with a broken bottle, ending up in court, where he was sentenced to six months in a young offenders’ institution.

Languishing at Her Majesty’s pleasure, Seb now has plenty of time to reflect on his first steps towards a life of crime. “The buzz I got when I stole that CD was massive, but that soon disappeared. All of a sudden I didn’t feel the gangster any more. What started off as a bit of a laugh with my mates is now dictating the rest of my life. It’s a living nightmare and one I pray to God you never have.”

A cautionary tale, you may think, and that’s the idea. Seb’s story is fiction, a play called Crime of Your Life, but it is based on a familiar pattern observed by police in Gwent, who commissioned the drama as a way of getting the anti-crime message across. The idea came from a conversation between PC Glyn Hopkins, an ex-teacher who is now school liaison officer for Gwent police, and local magistrate Gill Capper. They were looking for an alternative to the usual approach of classroom presentations by the agencies involved. Peter Williams, performing arts lecturer at Gwent college, wrote the play, and a team of students from his course make up the cast, which tours Gwent’s eight comprehensives.

For the play’s London debut, at Villiers high school in the London borough of Ealing, children at the school took the roles of Seb, his friends and parents. But the adults - police, prison officers, magistrates and so on - were all played by people who do those jobs in real life. They also run the workshops that follow the performance to reinforce the “crime doesn’t pay” message.

The story ends with Seb being “processed” as he begins his sentence, being bullied by other inmates, then being handed his regulation issue prison clothes. If the fiction is realistic enough, the facts of prison life, about to confront the children, are the stuff of nightmares.

Probation officer Bill Modrate describes the frightened, confused young men he meets daily as they arrive at Wormwood Scrubs. “When you are in prison they take away your family, they take away your friends, they take away your identity,” he says. “You are nothing, you are not even a name. They give you a number instead.”

“Mine was FB7228,” chips in the tracksuited figure beside him. Conrad Griffiths has recently finished a 16-month sentence inside Feltham young offenders’ institution. The children can ask Conrad any question they like - except the reason why he went to prison, to avoid getting into discussions about the offence.

After three months inside, Conrad got a job as a cleaner. “It is something you wouldn’t dream of doing at home, mopping a wing all day long,” he says. “But people fought for this job.” He describes the prison regime - two pairs of clean underpants a week, one phone between 90 inmates on the wing, 44 hours locked up in your cell over the weekend - in stark, groan-inducing detail. The children ask if there was bullying (“yes, but I wasn’t bullied”), what the food was like (“disgusting”), and whether being in prison has changed him. “It was a big turning point for me,” he confesses. “I had done something wrong and I got punished for it. Now I’m a changed person.” Conrad has been out of prison for six months and has found work as a karate teacher.

Measuring the deterrent effect of a project such as this is difficult, but there have already been some successes. After one presentation at a school in Gwent, three lads came up to Mr Hopkins, and confessed they were running with a gang. “They said, ‘Can we have a chat, we are involved in crime but we haven’t been caught and we need some help.’ When those three boys approached us in that school at least we were able to stop them at an early point. What a result that was.”

As well as the workshops, PC Hopkins and his colleague Juliet Murphy - another former teacher - have put together a pack of classroom materials, including games, quizzes and activities, covering a wide area of the curriculum that can be used for several weeks afterwards.

“We are just giving them the information so they can make an informed choice. That’s all you can do,” says Mr Hopkins. “There are always going to be kids who never become involved in crime and there are always going to some who, no matter what you say, will become involved. A large majority could go either way.”

The 50-minute performance gets the message across to that majority without being condescending or dull. Its realistic depiction of the judicial process is interspersed with commentary from the presenters of a satirical game show, Crime of Your Life, in which the losers walk away with nothing and the winners end up in prison. It also makes clear that the perpetrator of crime isn’t the only one affected. Seb’s girlfriend leaves him, his parents break up, blaming each other for his behaviour. The shopkeeper can’t face going back to work and has a nervous breakdown. Mr Hopkins says children who saw the play three years ago still talk to him about it when he goes back to the schools.

The performance at Villiers was set up by Diana Dishley, of the local Magistrates in the Community scheme, together with the school’s PSHE teacher, Dan Rosser. Ms Dishley hopes to see it performed at all the secondary schools in the borough, and eventually at schools throughout the UK. She is sure that in the long run, unlike crime, it will pay. “If we can stop just one young person from getting into a life of crime, it will have been worth it.”

A teaching pack including a script of Crime of Your Life is available for pound;50 from Juliet Murphy, Gwent police, 1-3 Cardiff Road, Newport, Gwent, NP20 2EH. Tel: 01633 245209. Villiers high school, Southall: email Dan Prosser on rr155@villiers.ealing-sch.uk. Magistrates in the Community: diana@dishleyr.freeserve.co.uk