Put up two fingers and make a rude sign," demands renowned double bassist and string teacher Rodney Slatford. It's a bizarre moment: six middle-aged adults, including two heads of county music and a world-famous percussionist, each balancing a huge double bass against their chest, each pretending the obscene gesture in their left hand has nothing to do with them.
Further embarrassment follows. "Can anyone tell me which finger we play the G with?" Mr Slatford asks his class, having shepherded them through an eight-note scale. Blank looks and blushing silence. "Try playing the F-sharp," he urges. The beginner bassists can't find the right place for their fingers on the strings, they can't find the right-sized rude gap between their fingers, they can't press down hard enough on the strings for their bows to make a sound.
Evelyn Glennie, one of the world's top solo percussionists, is the one undergoing this trauma, and she shakes her head despairingly. Trevor Barlow, saxophone and clarinet teacher and Cambridgeshire's head of woodwind, clenches his bow in his right hand. Poised above the strings, the fingers on his left hand tremble with tension. An edge appears in Mr Slatford's voice. "You've instantly forgotten everything we were saying about the bow hold," he informs Mr Barlow, before suggesting to Ms Glennie that she shift the position of her second finger. He sighs, as she moves the third finger instead.
Plenty of reputations are on the line here. Mr Slatford's six pupils, all professional music teachers, but none with experience of stringed instruments, have an hour to learn the basics of the double bass. As head of strings at the Royal Northern College of Music in Manchester, he has an hour to teach them. It is going to be a very near thing.
And the pressure will not end there. For this is the Grade One-a-thon, a charitable instrument of torture in which music teachers are sponsored by their pupils and friends to take a grade 1 music exam after one lesson and eight weeks' practice on an instrument they have never played before.
All the money raised will go towards children's hospices in East Anglia.
There are three in Cambridge, Norfolk and Suffolk, with annual running costs of pound;4 million.
One hundred and sixty Cambridgeshire music teachers - plus a sprinkling of guest stars such as Ms Glennie and oboist Nicholas Daniel, recently appointed Prince Consort Professor at the Royal College of Music - have gathered at St John's College school in Cambridge to learn their new instruments, all on the same January Sunday. The perilous logistics of supplying those instruments, the job of volunteer Nigel Fordham, owner of the Cambridge Pianoforte Centre, became clear on Saturday night, when four of the six promised harps failed to arrive. Somehow, via the music teachers' grapevine, by 9am on Sunday three replacement harps have turned up.
Joining them are 19 cellos, seven bassoons, eight trombones, nine clarinets, 10 trumpets, 13 French horns and an assortment of flugelhorns, fiddles and euphoniums. A key attraction of the Grade One-a-thon is that teachers can choose any instrument they like, and for various reasons, including professional usefulness, curiosity, availability, and comic impact - petite violin teacher Rachel Rigby is struggling to lift her tuba - they have.
"I had a strict piano teacher when I was six," explains Debbie Goddard, flute, guitar and piano teacher and a primary music specialist. "In the room where I had lessons was a boudoir grand piano and a harp. Every week I would walk past this harp to get to the piano, and my teacher would never let me touch it. I had the impression that I was her worst pupil, that I would never be worthy of touching her harp."
That was 40 years ago. As part of the Grade One-a-thon, Ms Goddard will have a harp in her music room for eight weeks. "When I went in for my lesson, I couldn't wipe the smile off my face. Just being in the same room as five harps was extraordinary. And I love it. I want to play it all morning, but I can't because your fingers start to hurt after 10 minutes. I think I'm worthy of it now."
The knuckle-rapping tradition of music teaching is rarely in evidence today, though the self-critical anxiety some teacher-learners carry with them, together with a certain amount of professional rivalry, definitely is. But as the lessons go on, what becomes apparent is the emotional and technical tightrope involved in teaching beginners in a hurry.
Do you risk embarrassing an individual or allow a fault to go uncorrected? Do you teach in tiny steps or put everything together, then work on getting it right? How perfectionist can you be? How many embouchure-collapsing giggles can a French horn teacher allow?
Faced with nine novice clarinettists, Joy Farrall, one of Britain's most successful exponents, and professor of clarinet at Cambridge's Guildhall school of music and drama, opts for one step at a time. "Sometimes people kick off and they can make a sound, then, unless it's really established, they may not be able to do it the next day," she says.
Forty-five minutes into her lesson, each student is still playing long, slow notes and trying not to squeak. They have not started the three compulsory scales and pieces for their exam. Among them is Chris Child, head of Elm CE primary school near Wisbech, who made the mistake of telling one of his visiting music teachers that anybody could get grade 1 in eight weeks. As a "bad guitar player who only reads primary school recorder music", he is here to eat his words. In fact, he nearly eats the reed of his daughter's clarinet, which is far too soft for him and sounding rather chewed. He obviously has strong lip and mouth muscles, Ms Farrall tells him.
She moves on to cello teacher Josephine Honner, who is attempting a B-flat.
No sound comes out. "That'sI very quiet," she says. "Don't worry about the fingers. Don't think about it. Just stick it in your mouth and blow."
"Ah, I'm thinking too much," says Ms Honner, having a revelation. She sticks the clarinet in her mouth and blows. A rich, deep, terrific sound comes out.
In every lesson the air is thick with similes. To play the trombone you make a finger gun and do Darth Vader impressions. Trumpet players blow into peashooters. Clarinet players blow out candles. Double bassists pretend to hold an orange. A cellist's bowing action should be pear-shaped.
These imaginative leaps help any beginner, but particularly children, get over the strangeness of instrumental technique, something which music teachers, in their 20, 30, 40, 50 or even 60-year familiarity with their own instrument, risk forgetting, says violin teacher and French horn novice Shona Clark. "The lesson was brilliant, but it does open your eyes to how a child must feel, particularly in front of other people. You're suddenly aware of how ridiculous you look, making funny faces and blowing raspberries, and at the same time you want to show the teacher that you can do it and you're really trying."
And that's not all there is to worry about. On Sunday, February 29, courtesy of a posse of volunteer Associated Board examiners travelling en masse to Cambridge, there are the grade 1 tests, followed by a public evening concert in which the teachers will play their new instruments.
No one - not even Evelyn Glennie - is obliged to reveal their exam scores, though some robust characters are promising mark-sheet copies to anyone who sponsors them for pound;10. The students - the teachers' real students, that is - love the idea. Many are sponsoring them. Several have offered to share their superior knowledge of their teacher's new instrument. One is even playing the piano accompaniment for the exam.
For they know what it's like to go through the confusion and self-consciousness, the squeaks and scrapings, the pressure and the tedious practice. And how will these teachers fit in the practice? "When my pupils don't turn up for their lessons," says Rachel Rigby wryly. "I shall buzz my lips in the car and in the bath and infuriate the neighbours," says cello teacher and novice trombonist Nicky Anderson, currently leading the sponsorship stakes with almost pound;1,000.
They don't face the professional performer's additional hurdle of travelling, as Ms Glennie points out. "Can you imagine being with the Strasbourg Symphony Orchestra and saying, 'Can I borrow your double bass to practise on, I have a grade 1 exam coming up?'"
But in every other way, says Nicholas Daniel, the experience of each instrument player, from the first lesson to the final concerto, is the same. "All this is about putting yourself in someone else's position," he says "That's what all teaching is about in the end. It's a leap of faith. I ache all over, I have gouges in my fingers, but it's been the most tremendous fun."
You can sponsor a Grade One-a-thon musician via the web pages at www.justgiving.com charityeach. Tickets for the February 29 concert at 7.45pm in the Guildhall, Cambridge, are available from the Cambridge Pianoforte Centre, tel: 01223 424007, or from East Anglia Children's Hospices, tel: 01480 465059. They cost pound;3 for adults, pound;1 for children