A new report has criticised the banding system which is used to select children for study at secondary schools.
Comprehensive Future has published a survey of admissions policies and finds that the system can lead to some of the most disadvantaged children losing out.
The group says there is often a "bewildering" range of such policies and a lack of consistency occurs even within the same region.
While systems such as banding have been introduced to promote fairness in schools' selection techniques, they rely on parents being organised enough to apply for the assessments and to ensure children attend them.
"This eliminates a whole tranche of children whose parents do not, for whatever reason, bring them to be tested and is likely to exclude some of the most deprived and disadvantaged," the report says.
Different selection criteria are often used by different institutions. Some select on the basis of faith or academic ability, while others favour aptitude at sport or music.
There are significant variations between local authorities, with some having large numbers of institutions that select according to tests or faith and others not. Even within the same local authority there is often a wide variation between schools.
Some schools have "extremely complex criteria", Comprehensive Future claims, which make it difficult for parents to predict whether their child will get a place in the institution.
The group predicts that as the number of academies increases, admissions criteria are likely to become more complex.
Comprehensive Future's secretary Margaret Tulloch said: "The more hoops children have to jump through to get into different schools, the more unfair the system becomes."
The organisation advocates a review of admissions practices and a standardisation of the banding tests across local authority areas to ensure children are selected fairly.
Conor Ryan, director of research at Sutton Trust, said fair banding and random allocation are the best means of selecting pupils for some institutions, as only the wealthiest parents can afford to live near some schools.
He pointed out that the UK's top 500 comprehensives have only half the average number of children on free school meals due to wealthy parents buying up property within their catchment areas.
Posted by Tim Colman