A "significant minority" of schools in England are breaking new laws that were designed to make the admissions system fairer, the government has said.
More than 500,000 children have been allocated school places
In a sample three areas it found parents were being asked for information they did not have to give, and even for money.
Schools Minister Jim Knight said it was "shocking" this was still going on.
Official figures showed 18.4% of children failed to get a place at their first choice school this year.
Some 94% were however offered a place at one of their three preferred schools.
The new, tougher admissions code came into practice in February last year.
Ministers asked officials to make the compliance checks.
EXAMPLES OF NON-COMPLIANCE
schools asking parents to commit to making financial contributions as a condition of admission
not giving looked-after children the priority required by law
asking about parents' marital, occupational or financial status
giving priority to family members who are not siblings attending the school
Manchester, Northamptonshire and Barnet were chosen simply as being representative of a metropolitan authority, a shire county and a London borough - and were areas where there had not been objections to admissions policies.
"We have no reason to think that these areas are any different from any other authorities around the country," said the Children, Schools and Families Secretary Ed Balls.
The large majority of schools appeared to be complying.
"However, a significant minority of schools in our sample appear not to be compliant with the code, of which a disproportionate number are voluntary aided or foundation schools."
The areas include 84 secondary schools and 486 primaries. Of those, 119 are voluntary aided or foundation schools controlling their own admissions.
Speaking to journalists, neither Mr Balls nor Mr Knight would put a figure on the number that were apparently acting illegally, pending verification.
But Mr Balls said: "When we saw the evidence we were very concerned. This was not a handful, it was certainly in the tens of schools."
Mr Knight said: "The fact that there are some things that are singled out in primary legislation that are still going on is shocking."
The schools were allegedly asking parents banned questions about their marital status, financial background or even asking for financial contributions to the school.
Mr Balls said that in a number of cases the financial requirement was "many hundreds of pounds per term".
His department later clarified that such financial demands involved "a handful" of schools.
"I want to be clear that this practice must stop immediately," he said.
Parents must be told the payments were voluntary and those who wanted their money back should be reimbursed.
Northamptonshire County Council's cabinet member for children and young people, Cllr Joan Kirkbride, said: "We were clear at the time of the new admissions code being introduced that schools would struggle with the timescale, particularly church schools which have to seek advice from their diocese."
The council had now ensured that every school was complying, ready for 2009 admissions.
She added: "We can categorically say that no school in Northamptonshire took money from parents. We can also say that the overwhelming number of schools who have been found to breach the code have done so in areas of minor technical detail."
The government is now going to amend a bill currently before Parliament to put a new duty on local authorities to report each year "on the legality, fairness and effectiveness of all school admission arrangements in their area".
Local admission forums, which are supposed to monitor arrangements, will be reviewed.
Mr Knight said he had spoken to "extremely senior figures" in the Anglican, Catholic and Jewish faiths and they were fully supportive of the government's actions.
Shadow schools minister Nick Gibb said: "All schools should abide strictly by the admissions code.
"But Ed Balls is distracting people from the real issue - that one in five children are being refused their first choice of secondary school."
The Association of School and College Leaders general secretary, John Dunford, said the new code was needlessly complicated and detailed.
"Compliance has been a complex process and schools and parents have found it equally confusing."
Andrew Copson of the British Humanist Association said: "Even we were shocked by the flagrant illegality of these schools' behaviour."
He added: "What further evidence does the government need to convince them that reform of state-funded faith schools is now essential?"