Parents face a school admissions system fraught with "confusion", according to a former adviser to Tony Blair.
Robert Hill says a minefield is opening up as schools are given the right to develop their own admissions policies.
This is creating inconsistencies as schools control their own admissions and over-subscription criteria.
He urged schools to work together to co-ordinate policies and ministers to reform the admissions code.
Mr Hill, who advised former education secretary Charles Clarke and Mr Blair, has written about the need for schools to collaborate rather than compete.
The book, Achieving More Together, has been published for the Association of School and College Leaders.
Speaking at its launch in London, he said: "The government is now pushing all schools to become foundation or trust schools.
"There would be a risk that the most vulnerable kids - either on ability or special educational needs - would be the ones who would lose out"
"In five years' time I wouldn't be surprised if the vast majority of secondary schools actually were foundation schools.
"Therefore we will have every secondary school as its own admissions authority, setting its own distinct over-subscription criteria.
"If they do that in isolation from each other, it is going to be very difficult for parents to understand."
He said confusion would arise because each school would have potentially very different arrangements.
"It would be a very confusing free-for-all," he added.
"There would be a risk that the most vulnerable kids - either on ability or special educational needs - would be the ones who would lose out."
Greater collaboration
Mr Hill is not suggesting that all schools should adopt the same admissions criteria, but he emphasises the need for schools to work in partnership to create a simpler and fairer system.
He also wants the government's statutory admissions code to be "much stronger" to encourage collaboration.
Another suggestion is that schools should be given extra funding for taking on children with poor academic results.
In the 1970s, local education authorities governed school admissions across their areas, setting the number of places in a school and deciding which schools children attended.
Under Mr Blair's 2006 Education and Inspections Act, all schools were invited to become foundation trusts, gaining control over their admissions arrangements.
"There is no evidence that a diverse set of schools will cause chaos or disadvantage any pupils"
Ministers billed the reforms as a means of giving schools much more freedom from local council control to develop their own individual ethos and raise standards.
Critics feared this would create a fragmented system in which schools compete for the best pupils at the expense of those from poorer backgrounds.
Many of the best state schools in affluent suburbs have become largely the preserve of families who can afford to buy expensive homes nearby.
The Department for Children, Schools and Families (DCSF) rejects claims that the new system is divisive.
A DCSF spokesman said: "There is no evidence that a diverse set of schools will cause chaos or disadvantage any pupils.
"On the contrary, evidence so far shows that parents are not having difficulty in applying to such schools."
Last week, schools minister Jim Knight warned some secondaries were still flouting new laws designed to give all parents a fair chance of winning places for their children.
Official complaints over admissions suggested some families were falling victim to unfair discrimination by schools and councils.
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