It reveals that some governors and local authorities are publishing one set of admissions criteria but using another when deciding whether to offer a child a place.
The disclosure from the three local government ombudsmen in England follows a rise in complaints about school admissions.
Latest figures show they dealt with more than 1,000 disputes over school places. They say that too many admissions policies are misleading, vague or unfair.
Their special report, just published, discloses that appeals panel members are not always independent and shows that in some cases they intervene in an attempt to persuade parents not to pursue complaints.
"We find too many examples of practice which is poor, sometimes spectacularly so," the report says.
One parent who appealed after being offered a place at a school she believed to have low standards and poor discipline was surprised to find that an appeals panel member was a governor at that school. The parent complained after the panel member intervened to defend the school and suggest it would be suitable for the child.
Another child was refused a place because the parent had not given the school as her first preference, even though this was not one of the admissions criteria.
One admissions authority was criticised for warning parents that "appeals can only succeed in the most exceptional of circumstances".
The ombudsmen investigate complaints of maladministration by councils, school governors and admission appeals panels. They can recommend that compensation be paid, pupils found places, or appeals re-run. There were 1,084 complaints in 200203 - up from 1,029 in the previous year. A third of complaints against faith and foundation schools, which set their own admissions criteria, were successful. The figure for community schools was 27 per cent. Their admissions are the responsibility of their local authority.
Some faith and foundation schools set criteria which favour children with articulate parents, the report says.
The report's authors warn against religious schools holding competitions among families to see who has the strongest faith, saying that criteria should be clear and objective.
Martin Rogers of the Education Network, a local-authority think-tank, called on the Government to strengthen parents' powers to challenge admission rules. "It really is time that all this stopped. If people did what they are supposed to, there would be less inequity in the system. By the time cases get to the adjudicator it is often too late for the parents."
School Admissions and Appeals is available from the Commission for Local Administration in England, 10th floor Millbank Tower, London SW1P 4QP