Some go further. Graham Thorpe, a senior researcher with the Scottish Council for Research in Education, six years ago analysed data from the Assessment of Achievement Programme and concluded that the benefits appear to be "incontrovertible" for switching all classes to composites.
Mr Thorpe said: "Assuming that each stage of school covers two years of ability, the added value in small composite classes is of the order of one to two months of schooling."
After looking at results from three different AAP surveys in English, mathematics and science, he came to the conclusion that pupils in small composite classes outperformed all others in P4 and P7.
Studies of Gaelic-medium education, which inevitably uses composites because of the smaller numbers involved, show strong positive effects in science and English but not in maths.
In an international literature review of composite classes, Valerie Wilson, director of the SCRE centre at Glasgow University, argues that such Scottish evidence should be considered cautiously but concludes that in Europe there is "no evidence to show that composite classes affect pupils'
academic performance adversely".
As Dutch researchers say, composites may "simply be no worse and simply no better". A key factor, Dr Wilson points out, could be that composites in Scotland tend to be smaller classes, which parents approve of.
Effects on teachers are more difficult to assess as there have been no studies in Scotland, although the common classroom view here and abroad is that composites increase workload and stress. Dr Wilson accepts that teaching in composites requires more planning and better organisation.
The reality north of the border, as it is around the world, is that composites exist because of small or shifting rolls in rural and urban communities. At the last official count in September 2001, 23 per cent of Scottish primary pupils were taught in mixed-age classes: 4,630 composites against 12,606 single-stage classes.
Nearly eight out of 10 Scottish primaries make use of composites at some stage.
Studies in North America are more positive about composites than in Europe.
Other factors are probably more important in influencing outcomes such as school organisation, the characteristics of teachers, teaching strategies, the curriculum and ways of measuring progress.
"All In Together? An overview of the literature on composite classes", by Valerie Wilson, was commissioned by the Scottish Executive.