Julia Barbour was painting an Airfix model of a battleship when she received a telephone call inviting her to compose a piece of music for an audience of 40 million people. As a schoolgirl of 14 who had composed only one tune in her life, she was more than a little excited.

The news that she was one of six winners of the BBC Young Composer of the Year competition — and would therefore be expected to write a fanfare for the Last Night of the Proms, which takes place tonight — left her so breathless that she could barely speak when she rang her mother. “When she heard me she thought there had been an accident because I was so frantic,” she said.

The GCSE student, who is now 15, is the beneficiary of a daring move by the BBC Proms. Winners of the Young Composer contest are usually rewarded with a performance of their music on a late night programme on Radio 3. Roger Wright, controller of the station, decided to raise the stakes this year by asking them to write pieces for the Last Night, which has the largest audience of any classical music event in the world.

Nicholas Heppel, Julia’s music teacher at King Edward V1 Handsworth School in Birmingham, arranged for her first composition to be recorded by the Birmingham Contemporary Music Group and mentioned airily to her that he had entered it in the competition. She thought no more about it until she received the telephone call. She named her piece, appropriately, After the Shock.

“This is the second thing I’ve ever written and it’s going to be premiered in front of millions and millions of people on live TV,” she said. “It’s too big to imagine.”

It will be performed as two fanfares, ten and 20 seconds long, as part of a series of calls and responses by orchestras performing on the Last Night in the Albert Hall and Hyde Park in central London, Buile Hill Park in Salford, Glasgow Green in Glasgow, Hillsborough Castle in County Down, and Singleton Park in Swansea. The other calls and responses have been written by Eoin Roe and Joseph Davies, both 16, and Lawrence Dunn, Sakura Tanaka and Aaron Parker, all 18.

The Times witnessed the winners listening to their music being performed for the first time at the Royal College of Music. Eoin, in particular, was nervous at hearing his music come to life. “I’ve had a bit of a block recently,” he said before his fanfares were performed by the Aurora Ensemble, a group of professional musicians. “I’m not very pleased with it. It’s passable, but I want it to be better.”

Eoin, who studies at Forest School in Walthamstow, East London, and attends the Guildhall School of Music and Drama on Saturdays, was assigned one of the most complicated responses, which had to involve five cornets, three tenor horns, two euphoniums, two tenor trombones and a tuba. “[My block] started earlier this week. I couldn’t find the harmony. It’s the first time I’ve written for brass.”

He grew in confidence throughout the afternoon, but is understandably daunted by the scale of the event. The largest audience for his previous compositions have been up to 100 people. “[The concert hall] was quite a big place, but not quite 40 million.”

Joseph, who studies at Chetham’s School of Music in Manchester, said that he tried not to think about the size of the audience. He hopes to pursue a career as a cellist rather than full-time composer. “It would be fantastic [to be back at the Last Night in the future] but I don’t think that’s going to happen.”

The most confident winner is Lawrence Dunn, who has formally been taught composition only over the last academic year but has studied on Saturdays at the Trinity College of Music junior department since the age of eight. He did not expect to win because his competition entry was “really mad and not particularly performable”, but he was commissioned to write the fanfare that will signal the beginning of the calls and responses. “The Last Night of the Proms is so magical, but it is scary. It seems like a grand old English institution that I don’t have the capacity to imagine. I feel as if I’m invading it.”

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