Some of the county's voluntary-aided and foundation schools may favour certain groups of children over others, according to a report by the Research and Information on State Education Trust (RISE).

In particular, they favour those children who already have a connection with the school, including children of staff and former pupils.

Schools in the region doubt the findings, saying pupils are selected irrespective of sex, family, siblings or background.

'External agency'

The report, which is based on analysis of admissions criteria, concludes there are opportunities for some schools to 'select in' and 'select out' certain pupils.

This enables them to obtain higher positions than others in league tables, say RISE.

Chris Beale, the assistant head of Pates Grammar School in Cheltenham, disputed the findings.

"We have to make absolutely sure that when we take children in, they all sit the same exam on the same day.

"Seven hundred children turn up on one Saturday in November and sit the same test set by an external agency, which then marks the test.

"Places are offered to the top 120 students irrespective of sex, family, siblings or background," he said.