Mr Everitt has found a variety of reasons for this volatility, the most compelling of which is the apparent shift in emphasis towards practical music-making in schools - an area in which many teachers feel ill-trained and ill-equipped.
This thought-provoking report echoes many teachers' worries about the status of music in the secondary syllabus; the subject is compulsory only as far as key stage 3 and not universally available at GCSE level.
Mr Everitt explores the problems encountered in schools where music is not a priority, and poses the question: "How are teachers to stimulate creative practice, usually in a single classroom and with insufficient equipment?" Neither of the two most common solutions is ideal. The first, to break the class down into small groups to work on projects, depends on a collaborative skill and clarity of approach which is difficult to achieve in over-crowded classrooms. The second option, whole-class teaching, Mr Everitt believes is beyond most teachers, who are more comfortable with one-to-one instrumental teaching.
Unless this fundamental problem is addressed, he writes, "the future of the music curriculum will be imperilled by poor educational outcomes".
Many non-musicians may question the educational value of music, but Mr Everitt shows that music-making is not just a rewarding skill, but can help foster personal and intellectual development.
He puts a convincing case for universal provision to be maintained up to and including key stage 4, and suggests that expanding music studies to include all secondary school children could arrest the post-school decline in music-making.
Mr Everitt points out that many state secondary schools have, since the introduction of local management of schools, found themselves deprived of the finance previously offered by local education authorities to provide free or heavily subsidised instrument tuition. By charging pupils, they have effectively excluded many children from low-income families.
Another worrying factor is the greater concentration of pupils learning to play the cheaper instruments, such as the clarinet and flute, rather than the bassoon or trombone.
Larry Westland CBE, executive director of Music for Youth, which offers a range of participatory events to young people throughout the country (including the National Festival of Music for Youth and the Schools Proms), says the situation is critical. "As there is no statutory right for children to receive instrumental music tuition in schools, it has become a situation of only those who can pay can play," he says.
Mr Westland says the report "seems to address a lot of fears and questions we have over the future of participatory music, especially in schools".
Joining In concludes by calling on the Qualifications and Curriculum Authority, in association with the Arts Council, to set up a working party drawn from teachers, music educationists, local authority representatives and musicians to find practical ways of helping to deliver the music curriculum and support music-in-education schemes and community schools.
"It is essential that the music curriculum is made to work effectively, " the report says. "This means ensuring that music is a part of children's lives from playgroup through to nursery, primary and secondary school."
Joining ln is available from the Calouste Gulbenkian Foundation, Turnaround Publisher Services. Tel: 0181 829 3000, price Pounds 10