Parental preference is causing admissions chaos in some areas, according to the Funding Agency for Schools.
Parents have a right to express a preference but some have unrealistic expectations about getting their children into popular schools - and ignore alternatives where they would have a better chance of getting a place.
The agency - which looks after grant-maintained schools and is to be abolished next spring - says that the solution is to give parents better information on the availability of places and the chances of getting one at a particular school.
But Margaret Tulloch of the Campaign for State Education said admissions problems had arisen in areas where schools have opted out and become partially selective - such as south-west Hertfordshire, where the Government has imposed a new admissions system.
She said: "If parents have got to make separate applications to six schools to get in, then better information is OK - but it's still a very stupid system.
"Opting out and being partially selective have given rise to a lot of problems. In certain areas, we have arrived at a very fragmented system which makes things more complicated."
The Government's guidance encourages grant-maintained schools and local education authorities to work together on common entry arrangements in local admission forums. A statutory code of practice, due early next year, will make the forums compulsory from September 2000.
The aim is to reduce the problems for parents in areas served by several admission authorities. The funding agency says there will be more than 150 such authorities in Essex and 30 or more in some smaller ones such as the London boroughs.
The agency believes its guidance, issued in response to the Government's draft admissions advice, comes too late for next year's intake as many secondary schools will publish their admissions information next month.
Unless shadow forums are set up this autumn, it will be too late to make joint arrangements for 2000, it warns. Schools and local authorities will need more guidance soon on how to set up and run such forums.
Michael Collier, the funding agency chief executive, added in a letter to the Department for Education and Employment: "The admission forums will have a key role and we do not underestimate the difficulty in such a body fully representing the views of all the interested parties within an education authority catchment area."
The National Union of Teachers and the Society of Education Officers acknowledge that long-standing differences over school admissions will not be easily overcome.
"It should be noted that the existing arrangements have encouraged a competitive relationship between admissions authorities and it may take more than simply the issuing of guidance to reverse that trend in some areas, " says the NUT's response to the guidance.
Andrew Collier, general secretary of the SEO, said the system for enforcing the new guidelines - relying on complaints to local adjudicators or the Secretary of State, who would make binding rulings - may not be "sufficiently swift or certain to eliminate bad practice".
He added: "The actions of a minority could do substantial damage to the credibility of fair admissions policies and strategies across the country. "