The government has been put on the spot by the news that the greater gains in top A-level grades in recent years have been in private and grammar schools.
The trend since A-levels were changed in 2002
Education ministers say the rise in the overall proportion getting an A - now 25.3% of entries - is down to to "better teaching and learning".
This raised the question of why state schools had not been doing so well.
Ministers said sustained investment in mainstream education was the way to increase opportunity for the many.
Looked at over the five years since the Curriculum 2000 changes to A-levels, the proportion of entries from independent schools awarded an A has increased by more than six percentage points.
In state grammar schools it was almost as great.
This was double the improvement in the majority of state schools and colleges, according to the Joint Council for Qualifications.
The differences were revealed by the director general of the biggest board, AQA, Mike Cresswell, in a briefing for journalists, to pre-empt suggestions from "the usual grumpy old people" that the exams must be getting easier.
"If that were the case you would expect to see students from all sorts of schools and colleges and backgrounds showing the same average increase in outcomes," he said.
"They sit the same exams, we mark them the same, our examiners don't even know which school or college they are from."
He said the improvements were down to improved teaching and learning. He was not going to speculate on why there were such differences between different centres.
"But those differences would not exist if the exams were getting easier."
Some people have questioned his conclusions.
The percentage improvement in private schools over the five years was 15.7%, whereas in comprehensives it was 17.6%.
So although the absolute increase in comprehensives was less than half that in the independents, they had improved at a slightly faster rate.
But then the gap between the two has widened, with the independent schools pulling further ahead.
'Selection helps the few'
Schools Minister Jim Knight said that overall the number of A-level entries had risen by 8% since 1997.
"This means that thousands more young people from all backgrounds are now taking and achieving A-level qualifications, which is something to celebrate."
He said the number of A grades achieved by pupils in state schools had risen between 1997 and 2006 and the independent sector's share of the total number of A grades had actually fallen over the same period.
"What this research actually demonstrates is that selection helps the few, but that sustained investment in mainstream education is the way to increase opportunity for the many.
"That is why we have increased investment in state schools, and it is why we are delighted with the record results achieved today."
The head of the Association of School and College Leaders, John Dunford, said it was hardly surprising that independent and selective schools had the better results.
"Their students have been rigorously selected, come from on average wealthier homes, and in the case of independent schools are taught in smaller classes and have more money spent on them.
"Selective schools having the best results is the educational equivalent of the Pope being a Catholic."
Dr Dunford challenged the Prime Minister, Gordon Brown, to fulfil the pledge he made as Chancellor to raise state school funding to independent levels.
The general secretary of the Independent Schools Council, Jonathan Shephard, said private schools attracted better qualified teachers than many state comprehensives.
In some state schools pupils were under peer pressure to behave badly rather than work hard, he said.
Another factor was the independent sector's freedom to innovate.
"The ability to teach children above and beyond the national curriculum - and at times to ignore the national curriculum - is very important," he said.
"It gives teachers at independent schools a greater degree of professional freedom and that does look to benefit the children."
The high master of St Paul's independent boys school, Martin Stephen, said he thought it was more about centres of excellence.
A concentration of bright students could reach "a critical mass", he said.
"They spark each other and create a culture of learning and in turn stimulate the teachers," he said.
"If you educate like with like you achieve a total outcome that's greater than the sum of the parts."
This did not only happen in independent schools, there were some excellent maintained schools too, Dr Stephen said.
But the problem in much of the state sector was a "one size fits all" philosophy about 11 to 16 education, which did not work - especially for gifted children, who needed to be seen as a "special needs" category.
"We have this extraordinary idea in the UK that one teacher fits all too. It doesn't work," he said.
Conversely, the greater improvements in the grade E pass rate have been in colleges, comprehensives and secondary modern schools.