The government has admitted that 800 schools are letting pupils down by failing to achieve basic exam standards in what amounts to an unacceptable "waste of talent and potential".
The admission from the schools minister, Lord Adonis, that a quarter of England's secondary schools are failing to meet the government's own targets for GCSE results prompted fury from teacher groups, who accused the minister of writing off good schools by changing the measure of a successful school.
Lord Adonis told a conference of private school heads today that less than 30% of 16 year olds at these schools were achieving five or more good GCSEs, including English and maths.
"The waste of talent and potential this represents simply isn't acceptable for the future," he said. "Parents rightly expect better, and so must we as educators and government." He added that while some were improving, others were "not improving fast enough to give parents confidence".
"For many of these schools, the essential deficit is one of governance, leadership, ethos and vision taken together - all of the underpinning foundations of a good school, which together enable it to succeed," he said.
The development of the state-funded academy schools, which are run by private sponsors with more independence than regular schools, was a direct response to these failures, he said.
The minister was speaking at the annual meeting of the Headmasters' and Headmistresses' Conference (HMC), a group of 250 leading fee-paying schools, where he was promoting ways for independent schools to take part in the academies programme.
In guidance issued by Lord Adonis to private-school heads, the academy programme is described as offering a "once-in-a-generation opportunity to break down the historic divide between the state and private sectors of education in England".
Becoming an academy offers students the chance to "work and learn together in an expanded, and often socially more mixed community", it says.
Birkenhead high school has become the latest fee-paying school to apply for academy status, and the guaranteed state funding that comes with it. Woodard Schools, the group that runs Lancing, Ardingly and Hurstpierpoint colleges in West Sussex, is sponsoring three academies in the area.
Last month, two other schools, William Hulme's grammar and Belvedere, made the transition to become academies.
In a move to allay fears that the academy programme will be seen as a system to secure the future of private schools that are financially struggling, Lord Adonis revealed that some schools that had proposed a switch had been rejected because they weren't of a high enough quality.
"We have had informal approaches, some of which we haven't wished to pursue further," he said.
Nearly 50 private schools have now agreed to sponsorship or partnership deals with academies - almost a quarter of the number of academies that are up and running or in the pipeline.
However, far fewer are seeking to become academies. Most of the schools that have made the transition had previously received state funding under direct-grant and assisted-places schemes, which were scrapped by the Labour government in the 1990s.
The general secretary of the National Union of Teachers, Steve Sinnott, called the move to encourage private-school sponsorship of academies "extraordinary".
"I reject the implication that somehow private schools and the quality of teaching within them are better than that in state schools," he said. "Day in, day out, teachers in schools in socially deprived areas make an enormous and positive contribution to the lives of their pupils.
"I have to wonder how private-school academies are going to get anywhere near that kind of commitment and experience."
He added: "My other concern is that cash-strapped private schools will be tempted into academy status, thus transferring their financial burdens to the state sector.
"There is every argument for positive links to be established between state and private schools. Encouraging private schools to run state schools will not help those partnerships."
