Commodity trading seems an improbable subject for a school concert. But that’s the theme of Arabica, the latest musical entertainment created by two Lancashire teachers, Peter Rose and Anne Conlon, to be performed by the pupils of St Augustine’s Roman Catholic High School in Billington near Blackburn, Lancashire.

If the track record of their earlier shows such as African Jigsaw and Ocean World is anything to go by, this too will reverberate around school halls across the world. Yanomamo, their piece about the threat to equatorial rain forests (first staged at St Augustine’s in 1983), has been performed more than 300 times by other schools from Barnsley and Billericay to Bombay, Florida, Germany, Hong Kong, Indonesia, Norway, Mexico and Yokohama.

It’s not hard to see why. Watch the choir and musicians of St Augustine’s in a disciplined Sunday afternoon rehearsal on home ground or in a spirited performance with all the resources of the Queen Elizabeth Hall at London’s South Bank and it’s obvious that the Rose-Conlon team writes in a way that challenges young performers into a genuine commitment.

They’ve been producing these concerts for 15 years now. The first, in 1981, was The Conversion Job, about St Augustine’s mission to the Angles. Amazingly, the most recent musical they staged at the school was Ocean World six years ago - which means that the current crop of performers started from scratch, with no previous experience. They took the stage like seasoned troupers.

Both Peter Rose and Anne Conlon are proud of the fact that a priest in Kenya has put on African Jigsaw working from a tape of their 1986 production. The St Augustine’s choir of 147 and the 28-piece orchestra are self-selected from the 1,000 pupils of mixed ability in the non-selective school. “You’ve got to be able to sing in tune,” Peter Rose says, “but nobody is told they can’t sing.”

Everybody says that Rose, who writes and directs the music, could have been a concert pianist and a first class one at that. But he started teaching while still a student at what is now the Royal Northern College of Music in Manchester and found more satisfaction in sharing his skills. “You get the feedback,” he says. Now head of music at St Augustine’s, a school where music was “limited” when he arrived in 1970, he asked Anne Conlon (a family friend who is senior teacher at SS John Fisher and Thomas More RC High School in Colne, just the other side of Pendle Hill) to write the libretto for his Augustinian musical. “Peter had written liturgical music for the church choir,” she says. “I had written a few very introverted poems as a student, like you do.”

But their second, six-minute, piece, “The Kestrel Song”, won a competition run by BBC Pebble Mill and the World Wide Fund for Nature, which led Ivan Hattingh of the WWF to commission a musical about threatened rain forests. In 1983 Yanomamo went on to the Edinburgh Festival and eventually took 200 Lancashire schoolchildren to Washington and New York, with rock star Sting as narrator. It was shown on television, too, as was their second WWF commission in 1986, African Jigsaw, about migration from the country to the city, and Ocean World in 1990, the story of a mother and baby whale, narrated by David Attenborough.

WWF sees these shows not only as gala benefit nights but as a way of involving young people in global environmental issues. This time Christian Aid has joined in by arranging for Peter Rose and Anne Conlon to visit Nicaragua to see how the coffee growers are struggling against the multinational conglomerates and getting a better deal by supplying fair trade outlets.

In Nicaragua, they met an English artist, Daniel Hopewell, who is working with homeless children in Esteli in northern Nicaragua. The children from St Augustine’s raised Pounds 1,000 to help the Nicaraguan street kids, who sent back large paintings illustrating the themes of the show to be displayed at the performances.

Given a brief of writing a musical about unfair global trading practices, Rose and Conlon chose to focus on coffee because it’s something that everybody can relate to. The most valuable traded commodity after petroleum, coffee is “portrayed in such a glamorous way,” says Anne Conlon. “It’s taken two years to write. There were so many ideas, it took a whole summer holiday to sift through them.”

So grew the latest Rose-Conlon musical Arabica - the story of the coffee bean and its 1,000-year development from accidental discovery by an Ethiopian goatherd, through its growth in economic importance, to the bitter enslavement of the growers to the international bankers. It’s a moving tale enlivened by the light and shade of Anne Conlon’s witty, compassionate script and Peter Rose’s astonishing variety of musical styles - from pastiche Handel to tango and barbershop.

The performers - aged from 11 to 16 and of all abilities - have been rehearsing for six hours every Sunday since November. It’s a punishing routine: as the Virgin and Child look down on the cleared dining hall, one soloist is in tears and gets a cuddle; another, disengaged, boy is sent out.

The ratio in the choir is about four girls to each boy and there are few brown faces, considering that we’re so near to the large Asian community in Blackburn. But this is east Lancashire’s rural fringe - the beautiful Ribble valley - and the school is, of course, Roman Catholic.

“They are learning,” Peter Rose says, “how much value to place on the importance of what they’re doing. We are setting a standard.” And it would, indeed, be condescending to let them get away with anything less than their best.

Even though it’s a concert performance, rather than a fully staged musical, drama teacher Eileen Proctor has come in today to choreograph the movements (which make all the difference between stasis and vitality). Assistant director Andrew MacLaren, also a musician, prowls supportively and the headmaster, Anthony McNamara, obligingly supplies the fair-traded coffee.

I wondered whether this degree of discipline led to over-optimistic expectations of careers as professional musicians. “Lots of them have gone on to A-level music,” Peter Rose says, “and old students are teaching music everywhere - but that’s not what it’s about.” Anne Conlon adds: “It’s the experience that’s enriching.”

But by the time the five bus-loads of children get to the Queen Elizabeth Hall for the scrubbed, shampooed, uniformed, slickly-lit performance with black actor Burt Caesar as narrator, they are indistinguishable from the real, paid thing.

The choir stands, sits, moves as one; their focus supports the soloists; their sound is massive, something we rarely get to hear these days - and for all the drilled precision, they display a vitality and exuberance and sheer delight in what they’re doing, which is irresistible. These children are not cowed, they emerge as dedicated professionals.

In spite of its curious genesis, a strange mix of marketing and idealism, Arabica is far from propagandist, more a down-to-earth plea for compassion in a corrupt world. It brings home the way that something so taken for granted as a cup of coffee affects the economy, society and ecology of the Third World; it stresses the interconnectedness of everything. It would be hard to disagree with the sentiments of Anne Conlon’s final number, underscored by Peter Rose’s soaring music, that “a fair world is a richer world” and “Each caring thought changes the world”.

Perhaps they won’t all boycott commercially traded brands of coffee - no doubt some of these children will soon be grappling with drugs and sex and all the other teenage preoccupations - but they have at least given some thought to the complex issues involved, and they have learned the satisfactions that come from prolonged co-operative effort.

They will go on to perform the moving climax again in Manchester and Blackburn; there’s also a special performance for a conference of bishops which has the head wondering where he’s going to find 27 thrones. As Peter Rose says: “It’s a long haul. The kids come back in their twenties and say it’s what they remember about school.”

St Augustine’s RC High School choir and orchestra will perform Arabica at the Palace Theatre, Manchester on June 23 and at King George’s Hall, Blackburn on June 29. Cassette recordings of Arabica and the other Rose-Conlon musicals performed by the school are available from Education Department, WWF-UK, Panda House, Weyside Park, Godalming, Surrey GU7 1XR (01483 426444). Sheet music and band parts are available from Josef Weinberger Ltd, 12-14 Mortimer Street, London W1N 7RD (O171 58O 2827).

* Richard Stilgoe’s Musical Masterclass on writing musicals will be published as part of TESSummer School, which begins on July 19