Nathan Leopold Jr and Richard Loeb were just 18 and 19 - precisely the ages now of Adam Chalk and Daniel Pirrie. On Tuesday, these two young men walked out on to the stage of the Arts Theatre in London, at the start of a very different trial: an attempt to recreate the roles of the notorious killers in front of a West End audience.

Never the Sinner, John Logan’s powerful recreation of the Leopold Loeb murder case, has been in London before, at the Playhouse in 1990. What makes the current production of this Chicago-based writer’s play remarkable is that it’s an amateur one, being performed by present and just-departed pupils of King’s College School, Wimbledon, a leading boys independent school.

So how did a school play come to have a three-week run at a West End theatre? Will there be sufficient punters to support the show once the pool of parents and loyal friends has dried up? And how are the young performers at the centre of this theatrical fairy story coping with being suddenly pushed into the limelight?

The story begins on the Edinburgh Fringe, that annual melting-pot of the innovative, the startling, the worthy and the inept. Few schools have the resources, human or financial, to take a show to the festival - but King’s College School has done so for the past four years.

Their track record is impressive. In 1993 they put on Pirandello’s Man, Beast and Virtue, while in 1994 they staged two productions: Athol Fugard’s My Children, My Africa! and Ben Brown’s Four-Letter Word. Last year they did another Fugard, The Island; this August it was Never the Sinner.

All five shows have been directed by Philip Swan, an English teacher at King’s. “The producer Paul Spyker saw the show in Edinburgh and was very moved by it,” he recalls. “He thought it was better than Rope (Patrick Hamilton’s play loosely based on the Leopold Loeb case, also recently shown in the West End).”

Co-producer Guy Chapman, who had not then seen the show, says he was initially hesitant about bringing the production into town. “I was very dubious at first, until Paul sent me a video of it. I thought the two boys were amazing, they seemed to have this rare naivete. There were then a couple of performances at the school, and I realised the production had to go on somewhere. That’s when the cheque books started to come out.”

Another attraction for him was the play’s exploration of the notion of justice and the death penalty. In his powerful summation Clarence Darrow, the boys’ attorney, pleads for understanding of the two young killers, suggesting that “mercy is the highest attribute of man”.

“He’s saying that humanity will change in the future, yet here we are in the mid-1990s, and capital punishment still survives in many countries,” adds Guy Chapman. “As with the film Dead Man Walking, the play adds to the debate about Death Row.”

This theme was also important for Mr Swan, an opponent of capital punishment. “Any play that makes you feel compassion for two such heartless boys must be a good one,” he suggests. “But it also touches on many aspects of the American judicial system that are still relevant, such as the way trials get turned into a media circus.”

I caught up with the production a few days before the company opened in their West End venue, while final rehearsals were taking place in the school’s own theatre in south London. “The cast are thrilled and excited by the idea of going into the West End,” Mr Swan admitted. At this stage they were concentrating on two problems: adjusting to performing on a traditional stage - previous performances in both Edinburgh and London had been in studio spaces with the audience on three sides - and introducing five new members of the cast in minor roles.

Most of the company, including the two principals, left school in the summer. The production includes one adult, English and divinity teacher Chris Day, who plays Clarence Darrow, the role famously taken by Orson Welles in the film version, Compulsion.

The changes since Edinburgh, with current pupils filling the vacancies, arose because some of the original cast were due to start at university. Somewhat bizarrely, one of them, Max Berendt, embarking on a drama course at Manchester University, was refused permission to remain in the show.

Never the Sinner is a good choice for a boys’ school, since almost all the characters are male. (Two girls from Wimbledon High School have been drafted in to play the two minor female roles.) It also helps that the staging is relatively straightforward, all the characters being on stage throughout.

But in other respects John Logan’s play is a real challenge for a director of a young cast. American accents need to be authentic; the frequent scene changes between courtroom and elsewhere have to be effected swiftly to keep the action flowing; and the homosexuality content needs to be handled with sensitivity. Above all, high-quality performances need to be coaxed out of the two principals.

With this in mind, Mr Swan initially rehearsed Adam Chalk and Daniel Pirrie on their own for two weeks, before bringing in the rest of the company. “It’s absolutely vital to get their relationship right, otherwise the play won’t work at all,” he observes. “I think there’s now an electricity of understanding between them.”

You don’t need to watch the two in action for many minutes to see what he’s getting at. Daniel Pirrie as Loeb is the one you notice first: quick-thinking, cocky, confident, ruthless, he dominates the action, moving naturally around the stage, remarkably at ease.

He’s confident off stage too, though it’s five years since he played a lead role at school. “I was nervous at first, but by the time we got to Edinburgh I didn’t feel like a schoolboy any more,” he says. “I don’t feel any pressure now, I don’t mind what the critics think, we’re just doing it for ourselves. ”

Adam Chalk also knows how to hold an audience. As the introverted, weak, dependent Leopold, obsessively in love with Loeb, he plays his part in more subtle style, able to convey the depth of an emotion through a single look, or a sudden moment of stillness.

“Playing such a person was tricky at first, because it goes against my character, which is basically outgoing,” Chalk says. “As for the West End, I don’t think it’s really hit me yet, and it probably won’t until we get into the theatre. It can be a pretty daunting thought, but I feel confident.”

While the producers and the Arts Theatre are taking most of the financial risks, the school was asked to subsidise the show to the tune of Pounds 5,000. Luckily a wealthy parent of one of the cast, who happens to be a professional “angel” (a financial backer of West End shows), came up with Pounds 3,000 while the school’s drama budget was raided for the balance.

On the publicity front, the producers have treated the show as if it were a professional one, carrying out the normal kind of marketing operation, with glossy posters, leaflets and postcards being distributed. Early booking, according to the box office, has been “buoyant”.

Mr Chapman, who was formerly involved with new writing at the Royal Court in London, has already produced two shows at the Arts this year. “We haven’t compromised in any way on this one,” he says. “It’s as professional as the other shows: it’s just a fabulous play that’s fabulously well acted.”

Any serious play must be welcome in a West End dominated by musicals, tired thrillers and Neil Simon. “As a piece of theatre it’s more enthralling than many things I’ve seen there,” says Philip Swan. “The quality of the acting is very high.”

The jury, in the form of critics and audience, are about to give a more impartial verdict on Never the Sinner. Should it be a positive one, perhaps other schools might be following in the footsteps of King’s College School? “I certainly think there’s a niche in the West End for this sort of production, ” Mr Chapman agrees.

Never the Sinner is at the Arts Theatre, London, until November 2. Box office O171-836 2132.