What is it?

The process of putting students into classes that have a mix of abilities, rather than streaming them into sets alongside others of a similar attainment level. The practice is still quite rare in England's schools (both primary and secondary), with most leaders opting for the more traditional approach of setting by ability, particularly in maths and English. Academics working on an ongoing research project entitled Best Practice in Grouping Students recently reported they had no problem recruiting 120 secondary schools that teach pupils in sets, but could only secure 17 that used mixed-ability grouping.

What do supporters say?

That the evidence is clear: low-attaining students suffer when they are streamed by ability. The Education Endowment Foundation concludes that "low-attaining learners who are set or streamed fall behind by one or two months per year, on average, when compared with the progress of similar students in classes with mixed ability groups". The research also suggests that there can be a long-term negative impacts on engagement and attitude to learning for lower-attaining students who are placed into ability sets. And such grouping has a negative impact on social mobility, with disproportionate numbers of disadvantaged children found in lower sets.

A 2014 research review from the Sutton Trust listed setting by ability as one of its examples of ineffective teaching practice. It said the approach can "create an exaggerated sense of within-group homogeneity and between-group heterogeneity in the teacher's mind", which could make them miss the mark with regards to differentiation. The concern, it continues, is that teachers may end up "overdoing their accommodations for different groups, going too fast with the high-ability groups and too slow with the low".

What do critics say?

That it is easier for teachers to differentiate effectively when they are dealing with a narrower range of ability. If there is a larger gap between the highest attaining and lowest-attaining in a class, pitching work towards the middle will mean that pupils are less likely to achieve because they will either be under- or over-challenged.

Former Ofsted chief Michael Wilshaw backed this argument. He referred to "the curse of mixed-ability classes without mixed-ability teaching", claiming that it holds back bright pupils from fulfilling their potential. Back in 2010, researcher Tom Burkard also told the Education Select Committee that poorly managed mixed-ability classes could be a factor in behaviour issues.

Evidence shows there's no denying that high-attaining pupils do benefit from ability streaming. The Education Endowment Foundation reports that, in direct contrast to their low-attaining peers, higher attainers "make between one and two additional months' progress when set or streamed compared to when taught in mixed-ability groups".

The simple fact of its long-running status in schools is also a factor. The survey responses from the Best Practice in Grouping Students project found that teachers and school leaders were concerned that a mixed-ability approach could be regarded as "unconventional", which might deter parents from choosing the school.

What now?

The debate is far from new – a literature review from the National Foundation for Educational Research says that streaming was becoming "increasingly unpopular by the 1960s" – and yet probably far from over. Research in favour of mixed-ability teaching continues to stack up, but will have to overcome a huge weight of tradition before being implemented in a significant way. A research paper published earlier this year identified the difficulties with this as "a vicious circle where schools are deterred by a paucity of exemplars and resources, and the educational climate is characterised as fearful, risk-averse and time-poor".