Popular schools have been accused of selecting pupils to boost exam results.
The Office of the Schools Adjudicator said this week that schools were continuing to choose children by ability or social group - despite repeated warnings that the practice contravenes admissions rules.
Dr Philip Hunter, the chief adjudicator, said in his annual report that 227 cases were handled by his office in the past year, including 139 complaints about schools using allegedly dubious selection policies to sift out weak pupils. Of these, 104 were upheld or partially upheld.
Complaints included schools discriminating against children from care homes, reserving priority places for teachers' children and using interviews and aptitude tests. The report also criticises faith schools for using unclear criteria to choose the most religious youngsters.
The conclusions follow the publication of a study last month that says England's top state schools are "socially selective" and fail to admit children from poor backgrounds. According to the Sutton Trust, only 3 per cent of pupils at the 200 schools with the best GCSE results qualify for free school meals - a key indicator of poverty - compared with 14.3 per cent of pupils nationally.
Dr Hunter said: "Adjudicators are still seeing too many cases where arrangements are not clear enough for parents to make a realistic assessment of their chances of getting places, or where schools are accused of selecting by ability or social group."
Critics fear last week's education white paper, promoting greater school autonomy and choice for parents, will lead to further abuse of the system.
Ministers promised last week that existing safeguards would be strengthened, with local councils given additional powers to maintain a fair distribution of pupils. But Professor Anne White, of the London School of Economics, said: "Schools have a vested interest in attracting children who are easier to teach and do well in examinations."
Professor White published research in 2003 which said that a "significant minority" of schools used dubious selection methods. This week she said:
"Adjudicators are only alerted to dodgy selection practices when complaints are made - in the large majority of cases schools get away with it."
Adjudicators dealt with 227 cases in the past year, 43 fewer than in 200304 but up on the 78 complaints made in 200102. A total of 35 complaints were made against schools failing to reserve places for children in social services care, and 25 were upheld.
Dr Hunter said rules would be drawn up to safeguard the rights of vulnerable children. The report said 24 objections were raised against schools giving priority to teachers' children, compared to 26 last year.
He said: "As last year, adjudicators upheld most of these or ruled that they should appear lower in the order of priority so that children of staff do not displace local children."
Adjudicators also criticised schools which reserve places for children who make them their first choice as the so-called "first preference first" rule should have been scrapped.
Hilary Wilce 22