Almost half of parents in some parts of England did not obtain their first choice of secondary school for their children, official figures reveal.
This year's admissions placing letters are about to be sent out
A breakdown of last year's admissions, obtained by the Conservatives, show success rates ranged from 100% to 51%.
They suggest that 100,000 families did not get their first-choice schools - though on average 93% did get a school on their list of preferences.
The statistics were provided by local authorities during March 2007.
Shadow children's secretary Michael Gove said: "These figures show that for large numbers of parents the idea that they can choose a school for their children is a myth. At the moment there aren't enough good school places to go round.
"The most acute problems are in inner city areas which means that it is the poorest children who miss out.
"We will give every parent in Britain the legal power to take their child out of a school they think is failing and apply to a New Academy.
"This will give all parents a meaningful choice of school and make schools and teachers accountable to parents."
Next Monday, 3 March, is this year's big "National Offer Day" when letters will go out to most youngsters moving up to secondary school, offering them a place for next September.
The government will be publishing statistics on preferences for the first time a week later.
But the Conservative Party has used the Freedom of Information Act to obtain what information was available from last year.
It comes with a caveat that it was produced as a result of an ad hoc request to local authorities.
Not all of them count the information in quite the same way, and 25 did not provide any at all.
Two of the areas that did do so operated an "equal preference" system so there were no "first" preferences there.
The Tories extrapolated from what was available to cover the missing data, using the average first preference success rate of 82%.
They say that, across the country, the best estimate of the number of parents who missed out on their first choice of secondary school last year was 100,572.
School Minister Jim Knight said that improvements in the school standards meant that there were more good schools available to parents.
"It's absurd to argue children will get a second-class education if they do not get into their preferred school," said Mr Knight.
"There are now only 17 schools where less than a quarter of pupils gain five good GCSEs, compared to 616 schools in 1997. Because there are far more good and improving schools across the country, parents have more options.
"Every parent already has the legal right and power to take their child out of any school and apply to any new school including an academy, so there is nothing new in what the Tories are proposing.
"This will not give any parent any more rights than they already have," said Mr Knight.