Children from working class backgrounds are being placed in lower sets than their ability merits, a study suggests.
Many schools set their pupils by ability for different subjects
Middle class pupils were more likely to be in higher sets, irrespective of their ability, research looking at 168 schools in England also discovered.
Half of the 10,000 pupils in the study were put in sets based on their ability - measured by test results at age 11.
The other half were grouped on a number of other issues, predominantly social class, an educational conference heard.
The study matters because earlier research suggests that pupils who are put in too low a set for their ability get worse GCSE results than those in the right set.
The findings will come as a blow to ministers who are keen to ensure that all pupils, no matter what their social background, are given an equal chance of succeeding.
A spokesman for the Department for Children, Schools and Families said the study it commissioned had only looked at a small number of schools and was not representative of the national picture.
The joint research team from the Universities of Manchester and Sussex aimed to look at how pupils in low attainment groups could best be taught.
Professor Judy Sebba said: "What we found was that school teachers, and in some cases pupils, think that they are set on the basis of ability or prior attainment - usually based on Key Stage 2 results.
"But when we analysed the data, we found that this was only true for half of pupils - the other half were grouped on a number of issues - predominantly social class.
"Where you have two children with the same prior attainment - the middle class pupil is more likely to be in the higher set and the working class pupil is more likely to be in the lower set."
This was likely to be down to a complex range of interconnecting factors, she said, adding that it raised issues which were implicit rather than explicit.
But she highlighted the fact that schools as a whole are likely to have a "middle class culture as an institution".
"Language, speech and all of that" will come into it along with parental pressure, she added.
Middle class parents are thought to understand the school system better and would therefore be more likely to push for their children to be taught in higher sets.
However, the team acknowledged that other factors such as teacher assessments, teacher judgements and pupil characteristics were also likely to influence set placements.
This was important because earlier research had suggested that pupils who are in lower sets than they should be get a whole grade lower at GCSE for the particular subject of the set, she added.
'Only by ability'
The research team said that schools needed to be clear about the method they were using to set pupils.
This needed to be communicated to parents and children.
And there was a need to review and modify placements in the light of these findings, they added.
The research, commissioned by then Department for Education and Skills, was presented at the British Educational Research Conference at the Institute of Education in the University of London on Friday.
The DCSF spokesman added: "We support setting by ability because it ensures that underperforming pupils have the most intensive support while stretching those with high potential.
"However, we have always been clear placing pupils in sets irrespective of prior attainment is not acceptable - setting should only be by ability."