Teaching pupils in mixed-ability groups is so unpopular that researchers struggled to find enough examples to complete a recent study into the practice.

They found that factors driving schools to avoid such classes included the “fear that parents may respond negatively and results will be endangered”, according to a new paper from the British Educational Research Association.

Academics at the UCL Institute of Education and Queen’s University, Belfast, sought to compare the test results of children taught in grouped ability classes against those taught in mixed attainment classes.

Mixed-attainment grouping is widely seen as difficult and unconventional

But they struggled even to find enough examples of true mixed-ability teaching to use in their study.

They had signed up 120 schools that teach pupils in sets, but found it impossible to find the much smaller school sample for mixed classes.

Their initial target included finding 20 schools with mixed-ability classes in the London area alone, a number which was eventually reduced to just 17 across the whole of England.

Dr Becky Taylor, the author of the paper, said the study showed “schools are generally reluctant to engage with mixed-attainment teaching, particularly in mathematics and even in an educational context that strongly advocates evidence-based practices”.

She added: “Mixed-attainment grouping is widely seen as difficult and unconventional, and therefore risky. It is student attainment outcomes which suffer as a result of this fear.”

Data suggested that teachers with little prior experience of mixed-attainment teaching, especially in maths, were unlikely to be involved in mixed-attainment teaching now.

Teachers also worried they would not have time to do the work needed to change their practice.

Sir Kevan Collins, chief executive of the Education Endowment Foundation, the organisation created by the government to promote evidence-based teaching, explained that while setting pupils by attainment could “produce some benefits for higher-attaining pupils”, it can also “hold back low-attaining learners by damaging their confidence and engagement with lessons”.

His foundation describes the practice of “setting or streaming” pupils as likely to have a “negative impact” on balance.

“Schools are right to ensure that their pupils are appropriately stretched but it is important that schools and teachers focus on the evidence,” said Sir Kevan.

“If they continue to group pupils by ability, they must carefully monitor the impact that it has on all their pupils.”