What you're going to hear is a piece of chamber music written by a boy who was not much older than you are now." Thus trading on three elements at once - Mozart's genius, his timeless accessibility, and the vast gulf between our educational expectations and those of the 18th century - the Emperor Quartet embarks on a lesson with 60 primary schoolchildren.
The music is graceful and springlike, and the audience is instantly hooked. When the children are asked afterwards if anything has surprised them, several express astonishment that four unamplified instruments should make so great a noise. How many of them are learning an instrument? A forest of hands shoots up: fertile soil.
The first violinist introduces himself; then the violist (by tradition every orchestra's also-ran) describes her role in enriching the texture; thecellist explains his duty to keep everybody tethered to the earth. As he begins to demonstrate this, his untethered spike slips from under him: mirth all round, and the ice is broken.
The musicians do an Irish reel, taking turns to play the melody, and invite groups of the audience to stand behind them and wave cards when their particular player is holding the tune: not so easy when the melodies and counter-melodies come so thick and fast.
"Now listen to this, and tell us afterwards what it might be used for." It is a Haydn minuet, stately in its demure seriousness, immaculately played. This is greeted by a hail of ideas from the floor: a posh tea party; a ball; music for calming people down. What clothes does it suggest? The hail redoubles: ball gowns, top hats, posh, posh, posh. When might it have been written? 1865? The 1970s? Then a girl says she is sure she heard it in Titanic, and others instantly agree. Okay, 1912, and musical history is rewritten.
The quartet then plays a Bartok piece full of glissandi and pizzicati. Invited once more to say what it suggests, the children start to think in dramatic terms: flying feet; somebody running away; a sword duel; a spider killing its prey. Then, after telling the story of the famous clown who inspired the piece, the musicians play Stravinsky's Little Titch. The whole hall raptly attends. Now it is time to incorporate some musical ideas. Using nothing but their hands, first with eyes open, then blind, the children learn the meaning of crescendo and diminuendo, and the importance of listening for cues from fellow players. Finally, they take it in turns to play the instruments themselves.
"But how can I play if I haven't been taught?" asks one boy reasonably.
"We'll help you," says the second violinist.
Group after group find themselves playing drones unaided, while the cellist plays the melody. This could go on for hours, because everyone wants a go. There is a real frustration when time is called.
There are smiles from the pair running the video camera. Simon and Pamela Majaro, whose Cavatina Trust has brought players and audience together, are visionary activists in the traditional Hampstead mould. "We're music lovers and amateur players," says Simon. "And we've long been ging to the Wigmore Hall, where we've become increasingly saddened by the ageing of the audience. So we decided to create a scheme to bring younger listeners in. That was the seed of the Cavatina Chamber Music Trust."
Malorees junior school, in the London borough of Brent, where this event is taking place, is one of several schools linked to their two-stage scheme, first getting a visit from a professional group (free for state schools), then being bussed to a concert at London's Wigmore Hall.
"At first schools were suspicious," says Pamela Majaro. "They felt there had to be a catch in a scheme which offered something for nothing. But now they're queuing up to join in."
Cavatina organises half-price tickets for the Wigmore Hall, and free tickets for selected concerts at the Conway Hall. The Majaros claim that the scheme is already inducing teenagers to go back to the Wigmore under their own steam. Today's event with junior pupils was preceded by a session with the Malorees infants section; the top-flight chamber ensembles which take it in turns to work with the trust also make curriculum-orientated visits to secondary schools, where they adopt a more analytical approach.
The Emperor Quartet, which is rapidly making a name for itself on the international touring circuit, regards this work as more than mere fun.
"It's in all our interests that these children should develop a taste for chamber music," says the cellist. "They are our future audience."
But the Government is not exactly stacking the cards in that audience's favour. Malorees school has, in Tom Rainbow, an exceptionally enthusiastic head of music who oversees its orchestra, wind band, recorder ensemble, and four-part choir; more than half its 240 pupils are learning an instrument. But the ever-expanding demands of the national curriculum now mean that all Tom Rainbow can squeeze from the timetable is a half-hour's music per child per week.
"It's being pushed inexorably to the sidelines," he comments. They have twice tried to enter the Schools Prom, and twice had to withdraw because they couldn't devote the necessary time.
At the Schools Prom, director Larry Westland is sympathetic, while realistically accepting that core subjects should have precedence even over something so dear to his heart as music. On the other hand, he is vitriolic about the Government's and Arts Council's blindness to the need for more money to be channelled into instrumental teaching. The much-vaunted "Pied Pipers" of the new National Foundation for Youth Music are no solution, he insists.
"Getting trendy characters rushing into schools shouting 'Come on kids, let's do it!' is nothing more than a temporary buzz. It's not how real musicianship is spotted and developed, or how a real love of music is born."
The Majaros are working towards a nationwide network of Cavatina-style trusts; they are already planning collaborations with venues that include the South Bank Centre and Birmingham's Symphony Hall. As Larry Westland points out, this can never take the place of national investment in music education, but while we wait for the Governmental penny to drop, it is a lot better than nothing.
Michael Church is a music critic