An evaluation of trials of the Wider Opportunities Pledge, aimed at key stage 2 pupils, found that some lessons were confused and it was unclear what pupils were being taught.
The report, by the Office for Standards in Education, of 12 local education authorities piloting the scheme found work of a consistently high quality in seven. In the other five there was evidence of good and sometimes very good features, but not consistently.
Over time, the report said, all pupils in KS2 "should have access to a free period of specialist instrumental tuition, wherever possible, lasting for at least a year".
In some cases inspectors found that not enough time during sessions was devoted to playing instruments.
Tutors had low expectations of their pupils and did not pay sufficient attention to posture, tuning and listening skills.
Inspectors also found that teachers misjudged the pace at which children learned and often introduced new aspects of tuition before old ones had been mastered and consolidated.
The Wider Opportunities Pledge was announced in 2000 by the then education secretary David Blunkett, who said that "over time, all pupils in primary schools who wish to will have the opportunity to learn a musical instrument".
All 12 pilot programmes involve three strands of learning. Children can expect to be offered new musical experiences, taught music skills and given specialist instrument tuition.
The best results were achieved when the strands were integrated, especially when musical skills programmes and specialist tuition were delivered simultaneously. The best programmes had pupils engaging in music-making throughout the sessions, and children had high expectations.
Inspectors found that there was a higher take-up of instrument learning where it was introduced in whole-class or large groups. In some cases every child wanted to learn to play.
The scheme has been piloted in schools in Barking and Dagenham, Croydon, Devon, Haringey, Hertfordshire, Kirklees, Manchester, Newham, Norfolk, Northamptonshire, Oxfordshire and Portsmouth.
David Miliband, minister for school standards, announced this week that local authority music services would receive an extra pound;1.5 million to expand successful work from Wider Opportunities to other schools.
Mr Miliband and Estelle Morris, arts minister and former education secretary, are working with musicians and teachers on creating a "music manifesto" to improve the subject in schools.