"Remember, guys, you're pretty mean machines," says Jeremy James Taylor (co-directing with Russell Labey), stepping in to stir up a little more testosterone. "You won't be talking about Mozart much in the breaks: you speak with your fists."
The boys don boxing gloves, lift a few weights, perfect their toughest expressions. When they start to sing - the classic "So You Wanna Be A Boxer" - the sound is raw, gutsy and completely compelling.
This will be the first time the National Youth Music Theatre has performed in the West End, in a run that begins at the Queen's Theatre tomorrow and continues until January 10. The company, which draws its amateur talent from schools around the country, celebrates its 21st anniversary this year, but until now has fought shy of putting on what might seem an obvious show for a youthful cast.
Alan Parker's rollicking spoof of early gangster movies, featuring splurge-gun-toting gangsters and dreamy showgirls, was a hit film in the 1970s, shooting Jodie Foster to fame in the role of Tallulah. Under pressure to make a version available to schools and colleges, Parker later put the film down on paper. But Jeremy James Taylor, NYMT artistic director, was never quite satisfied with the multiplicity of quick scenes and laborious scene-changes, and gained Parker's permission to modify it. The NYMT stage version of Bugsy is altogether fleshier, with a bit more rhythm, a bit more shape, and even some new music added by the composer of the original score, Paul Williams. This version made its first appearance at the Edinburgh Festival in 1996, showed at the Lyric, Hammersmith, earlier this year, and is still undergoing alteration as it heads for the West End.
Some of the present cast of 11 to 16-year-olds are old troupers from previous Bugsy productions, many are newcomers from around the country, including a preponderance of Londoners this time around. Under-16s are only permitted by the licensing laws to do four performances a week, and have to fit in 15 hours' school work, so this means a double cast, of 84 in all, for a commercial run. The majority of the actors are from state schools, virtually none from specialised theatre schools.
"These are real people, with dirty fingernails and joie de vivre," says Taylor.
Back in the rehearsal room, after only an hour and a half, the boxing scene is falling impressively into shape. Bugsy, played this morning by a diminutive and winning 11-year-old, Bobby Bethell (he shares the role with Michael Sturges), is trying to make a quick buck out of Leroy Smith's (Nana Kumi) boxing talents, by getting him into training at Cagey Joe's Rudolph Gordon gym. Already there is strong characterisation here, machismo, humour, and a tremendous energy.
Kaye Shepherd, the choreographer, shouts at them to keep their voices as low as they can. But she is pleased with the way things are going: "Boys don't box much these days, and they can be a bit mimsy when it comes to this scene, " she says, as they break for lunch. "But these boys seem to have understood it, they're getting the right mood."
Keenness and an appetite for work are the hallmarks of these young amateurs. "With professional actors, there's a lot of clock watching, but the difference with this lot is that they don't want to stop rehearsing," says Taylor. "We've got two weeks of rehearsals, and professionals say, 'that's impossible, you'll never do it.' But I say to them, 'You just watch: these kids are phenomenal. ' " The competition to be in this show was intense, with 10 or more hopefuls for every part. With the help of a tape, and four days of musical training in the summer holidays, the cast arrives at the first rehearsal knowing all the words, all the music. Many will have had dance or singing lessons for some time - such as the accomplished band of female tap-dancers learning a new "speakeasy" routine this morning in an adjoining room. But others are picking up the dancing from scratch.
The big improvement in the quality of drama teaching in schools, Taylor says, has made these youngsters much easier to work with than in the past because they don't need to be taught so much.
"Our main task is to give them a bit of discipline, and to get them to combine their individual skills into working as an ensemble. There are no stars in this show - the word is banned. Whether you're a silent newspaper-seller or Bugsy Malone, everyone has to learn the importance of everyone else."
Another significant change in the last four or five years has been the explosion of interest in male dance - inspired by figures such as Michael Jackson - making many boys see it as something macho and desirable, rather than effete and sissy. More than half the boys in this cast have dance lessons, including ballet - which they do for good posture.
"We've got dancers, movers and some who are completely raw," says Kaye Shepherd, who admits she finds the boys more interesting to work with than the girls. "The boys are so enthusiastic, they're more open, and easier to get to know quickly. Girls carry more baggage with them. They also tend to be more highly trained, so it's more a case of just telling them what to do."
Discipline, she says, is not a problem, because of peer pressure. "If someone's being too noisy or messing about, they'll usually feel a vibe from the others to knuckle down. They'll also take each other off and practise dance steps, if someone is finding it difficult."
While probably all of them would tell you now that they want careers on the stage, a good number will no doubt find their way there, with a few even hitting the big time. Jude Law, for instance, featuring in the film Wilde as the young Lord Alfred Douglas ("Bosie"), came up through the NYMT. With other NYMT alumni including Becky Lock, Helen Way, Johnny Miller and Julie Alanah Brighten, he will be taking part in the NYMT's 21st birthday gala concert, at the Palace Theatre on Sunday.
But 21 years ago, the National Youth Music Theatre came into being more by chance than by design. Jeremy James Taylor, about to take up an assistant directorship at the Young Vic, was suddenly called on to help a teacher friend at Tiffin School, Kingston-upon-Thames, and despite not having worked with young people before, found he thoroughly enjoyed it. When, a few years later, they put together a ballad about Elizabethan child actors, inspired by a poem by Ben Jonson, he persuaded the headteacher that they should take it to the Edinburgh Fringe.
"At first he thought I was mad - but the show was a success, people took it seriously and it won a Fringe award."
The Ballad of Salomon Pavey then came to the Young Vic in 1977, as part of the Queen's Silver Jubilee Celebrations, and from there developed the practice of drawing in young talent from around the country. Since 1976, the NYMT's achievements include 49 productions in Edinburgh, 27 new music theatre commissions, eight foreign tours, six seasons at Sadler's Wells, and even a short run on Broadway.
In 1993, Sir Andrew Lloyd Webber, as he then was, became its principal supporter, shouldering the organisation's administrative costs. The Department for Education and Employment has recently made a grant to develop the NYMT's community and outreach work, and a National Lottery bid is under consideration. Otherwise, however, the NYMT still has to rely purely on patronage.
More than 2,000 young actors, singers, dancers and musicians are auditioning for NYMT productions next year. There are also regular residential workshops, and "work experience" opportunities, too, for those interested in technical or backstage work.
For the cast of Bugsy, meanwhile, with only two weeks before the curtain goes up at the Queen's Theatre, the excitement and sense of adventure is all-pervasive.
"I just enjoy performing and being on stage," says Hannah Spearritt, playing Dandy Dan's moll, Louella, and one of the six 16-year-olds in the show who are sharing a flat for the duration. "It's what I've always wanted to do - it gives you a buzz being up there."
"But we are going to be sensible," adds Sheridan Smith (Tallulah), who is taking theatre studies, drama and media studies A-levels at a sixth-form college in Scunthorpe. "In our spare time we're going to be catching up with our college work."
For the under-16s, many with GCSEs looming, those living in London will fit in what school they can, while those from further afield are getting work sent to them.
David Parker, who plays Smolsky and a "splurged guest", is from the Lake District, but will be staying with another of the cast, "So I've got to keep the working going. I'll be set work for when I'm down here, and I'll just have to catch up. It'll be tiring, but it's worth it. I want to go into an acting career, and to say I've been in the West End will look great on my CV."
Bugsy Malone, Queen's Theatre, Shaftesbury Avenue, London W1, November 15 to January 10 (official opening and press night November 19). Box office: 0171 494 5040 For more information about the NYMT: 0171 734 7478. Extracts from Bugsy Malone can be seen on Children in Need on November 21 at 7pm (BBC1).