England's top-rated state secondary comprehensives "cream off" the most academic pupils, a study suggests.
Segregation continues in England's comprehensives
The pattern has emerged because of a "lack of choice", say researchers from the Centre for Economic Performance, based in London.
The best-performing schools tend to be in middle class areas and the worst in the poorest, according to the report.
The government denies that schools are segregated by class and says fewer schools are "causing concern".
There is a huge discrepancy in the average ability of pupils starting at the top performing state schools and those in poorly performing schools, the research suggests.
It notes that schools serve different neighbourhoods and these neighbourhoods contain pupils from very different socio-economic backgrounds.
The report's authors Stephen Gibbons and Shqiponja Telhaj said: "All the children in a rich neighbourhood go to school together and all the children in a poor neighbourhood go to school together.
"Such differences arise not because of the freedom for parents and pupils to choose schools, but because of a lack of choice given where a pupil lives."
The best performing schools are those in middle class areas, according to analysis of data between 1996 and 2002.
Segregation is even greater where schools can choose pupils according to ability, as in the case of grammar schools, or religious ethos, as in faith schools.
The report concludes that the degree of classroom segregation between children from rich and poor backgrounds has not changed since the mid 1990s.
They said: "There are wide inequalities in intake between schools, but there has been no systematic change over the period of our analysis."
The report is based on analysing the results of national tests taken at the end of primary school at age 11, and tracking which pupils were admitted to various secondary schools.
It found the best comprehensive schools took a disproportionately high number of children who did well in these tests.
The Department for Children, Schools and Families (DCSF) has denied that schools are segregated by class or prior attainment or "anything else".
A DCSF spokesman said: "It's a fundamental principle of our system that parents can express preferences between schools and that they should have the best possible information on which to base those choices.
"Performance tables, Ofsted reports, schools and local authorities themselves ensure that there is no secret garden of hidden information.
"That includes information about the prior attainment of pupils in any particular school, both absolutely and relative to the characteristics of the intake.
"With the exception of the few remaining grammar schools, we don't operate selection by ability."
The DCSF emphasised that all schools now deliver enormously improved results, compared to a few years ago.
Their spokesman said: "There are fewer schools than ever causing serious concern and the turn around is faster. The many new schools, including academies, are delivering excellent results while social and poverty gaps in educational attainment are narrowing - even though we acknowledge there is more to do.
"In short, parents can be more confident than ever that their child will get a good education, whatever school they choose."