A survey by Lorna Hamilton of Edinburgh University found that some schools which did not group classes according to ability were held back by staffing or restrictions on class space.
There is also evidence that some schools which set by ability in middle or upper primary plan to move the practice down into infant classes.
Others operated mixed-ability classes because they were unconvinced that setting was effective in educational terms and because they were concerned that damaged self-esteem in lower cohorts was "too high a price to pay".
The study, which received responses from 338 schools with rolls of more than 100, found that teachers who taught ability-grouped classes felt this allowed their teaching to be more direct and interactive with more time available for individual support. They also reported better discipline, reduced teacher workload and better morale.
Most schools suggested that pupil motivation, attitude and self-esteem were improved through setting, although they stressed the need for an appropriate ethos in the classroom and school supporting achievement for all. The most important influence cited for choosing this method of class organisation was the use of team teaching.
While the vast majority of schools reported a positive impact on motivation and attitudes to learning, there was concern for children in lower groups who might be locked into a situation they struggled to escape from. They were aware of the hierarchical nature of the sets and a few were said to "sense failure".
Dr Hamilton also found that the use of setting or broad-banding in primary has had little impact as yet on arrangements in S1 - thus raising fresh concerns about transition arrangements and the "fresh start".
Most headteachers felt that the use of setting and broad-banding in primary had little or no effect on placement in classes in secondary and that information was often not taken on board. Some expressed concern that the pace and challenge established in primary through the use of setting and broad-banding was lost as children moved to secondary and faced much less differentiation.
Headteachers reported that national test results were the most important criteria for deciding the composition of set or broad-banded classes, followed by teacher observations and judgments.
There appeared to be no parent concerns around setting or broad-banding because it was accepted as "standard practice".
Dr Hamilton found that setting, a more restrictive form of class organisation, is far more prevalent than broad-banding.
Ronnie Smith, general secretary of the Educational Institute of Scotland, agreed with Dr Hamilton that in-depth longitudinal studies should be carried out into the impact of setting and broad-banding policies in primary.
Mr Smith said that, if possible, he would like to see comparisons between classes where setting was used and those using mixed-ability class organisation.
* 72 per cent of the total respondents were currently using or had previously used setting or broad-banding.
* 54 per cent of those were intending to continue with it in future.
* 56 per cent who used setting or broad-banding did so in middle and upper primary, with 18 per cent planning to extend it to other stages; 36 per cent used it in infants, middle and upper; and 76 per cent used it for maths and 34 per cent for language.
* 86 per cent reported that the initiative had come from within their own primary school and not from any external pressure.