Hundreds of thousands of families in England are being told where their children will be going to secondary school in September.
More than 500,000 children have been allocated school places
Early reports claim that an increasing proportion have failed to get a place at their first preference school.
Schools Minister Jim Knight has told disappointed parents that they can appeal against the decision.
But head teachers are warning this is a drain on schools' time and may be "raising false expectations".
The parents of more than half a million children across England have been receiving letters giving details of whether they have been successful in getting their first preference school.
Early indications from polls carried out by newspapers, using samples of local authorities, suggest that more families have been turned down from their first choices than last year.
Jim Knight's call for parents to appeal has annoyed head teachers
Last year, an estimated 100,000 families did not get their first option.
There were very wide local variations - in some inner-city areas only about half of parents achieved their first choice, while elsewhere almost every family was successful.
But the Department for Children, Schools and Families says these were unconfirmed figures - and that official results will be published next week.
In Brighton, where a tie-break lottery system has been introduced, an independent school has announced it is introducing extra classes to accommodate demand from disappointed parents.
"What the lottery has done is to prompt parents to do something that they may not have done before: visit an independent school," said Richard Cairns, head of Brighton College.
Head teachers' leader John Dunford has warned that the higher-profile admissions system is creating unrealistic expectations of choice among parents.
And he criticised comments from the schools minister, that disappointed parents should use the appeals process.
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"It will be extremely unfortunate if parents respond to the minister's call to appeal," said Dr Dunford, general secretary of the Association of School and College Leaders.
"Not only will the vast majority be disappointed by the end of the process, a vast amount of bureaucracy will have been created and heads wasted hundreds of hours."
Mr Dunford also said that the creation of a nation-wide admissions day had "raised the stakes" - with more parents now expecting to be able to make a choice in schools and more likely to feel let down.
A spokesman for Mr Knight said that the minister was spelling out the existing position - "that if parents think they have a strong case, they have a right to appeal".
And he said the bigger picture was that a large majority of parents were successful in getting their first preference schools - and that the number of low-achieving schools was diminishing.
"But there will always be schools which are oversubscribed - and there will have to be a limit to how many pupils a popular school can accept," he said.
The ATL teachers' union criticised the way that the school admissions system appeared to provide school choice, when this was not really on offer.
"The vocabulary of choice is a fantasy and creates totally false expectations.
"It would be fairer to everyone to end this myth and let schools get on with providing the best education they can for all their pupils," said Martin Johnson, the union's acting deputy general secretary.