Some academy schools may be "manipulating" admissions to select and exclude certain pupils, a new report suggests.
A study by the Academies Commission says that there are concerns that the rise in schools taking on academy status could fuel social segregation, rather than reduce it.
It warns that some academies may be "covertly" selecting pupils by using extra information on families when making admissions decisions, or holding social events with prospective parents.
The concerns are raised in a new report by the independent Academies Commission, which is examining the impact on standards of more schools becoming academies.
The Commission said it had received evidence and research that some popular schools, including academies, are attempting to select and exclude pupils.
It says that this practice is not new, but the fact that academies have autonomy over their admissions has "attracted controversy and fuelled concerns that the growth of academies may entrench rather than mitigate social inequalities".
The Commission said it had heard examples of some academies "willing to take a 'low road' approach to school improvement by manipulating admissions rather than by exercising strong leadership".
Under the current system, all state schools must abide by an admissions code, which says they must admit pupils in a fair and reasonable way.
But the Commission's report said it had received numerous submissions suggesting that "academies are finding methods to select covertly".
The admissions code says that schools cannot interview children or parents, or give priority to youngsters whose parents offer it financial or practical support.
"Some witnesses suggested to the Commission that schools, including academies, have ways to get around this, such as by holding 'social' events with prospective parents or pre-admission meetings."
Schools and academies can ask prospective families to fill in a supplementary information form (SIF) when making an application for a place.
The report says that research shows that some schools, particularly those in charge of their own admissions, were asking for parents to fill in lengthy forms, involving open questions, and sometimes asking for information not allowed under the admissions code.
"Such practices can enable schools to select pupils from more privileged families where parents have the requisite cultural capital to complete the SIF in ways that will increase their child's chances," the report said.
It warns that as more school become academies, and in charge of their own admissions, "there is a risk that admissions 'game playing' may be extended further".
Academies were first set up under Tony Blair's Labour government, with the aim of raising standards in under-performing schools in disadvantaged areas.
The coalition Government opened up the programme in 2010 to allow good and outstanding state schools to convert to academy status.
More than half of secondary schools in England are now academies, with the numbers expected to rise further.
A Department for Education spokesman said: "All admissions authorities - be they local councils or self-governing schools including academies - must comply with our new fair admissions code. It is clear about what they must do when admitting pupils.
"We specifically changed the law so that anyone who has concerns about how any state-funded school is admitting pupils can formally object to the Office of the Schools Adjudicator.
"This Government has also given all good schools, including academies, the freedom to take more pupils - so increasing the number of pupils who getting a first-class education."
Shadow education secretary Stephen Twigg said: "This report highlights serious problems with Michael Gove's management of one of Labour's key school improvement programmes.
"Academies under Labour were about raising standards and this Government is putting that legacy at risk. The report issues a clear warning on the implementation of the academies policy, echoing Labour's concerns that under this Government the schools system is becoming chaotic, impacting on standards and fairness."
The Academies Commission was set up by the Pearson think-tank and the RSA.
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