I never understood why high spending on education was considered to be a no-no when local authorities did it, but highly desirable when private schools did it. So it is intriguing to find John Marks, of the Centre for Policy Studies, suggesting that private-school parents might be better off sending their children to state schools since they are more cost-efficient. Although independent schools get better results, an A grade at A-level, according to Marks, costs pound;3,000 more at a private school than in a state school sixth form.

But John Marks is touchingly naive. What he seems not to realise is that those parents aren’t just buying top exam results by sending their children to independent schools. They are buying status, a particular set of friends and contacts for their children, and a way of life. A key task for the schools is to make sure that most of their clients emerge at exactly the same social level at which they went in. It would be amusing to try and calculate the “value-added” achieved by Eton or Winchester or Westminster. It’s a meaningless concept when the only possible form of social mobility is downwards. Children arrive bright, well-prepared, highly-motivated (by and large), with supportive parents and bags of cultural capital. They then get half as much again spent on them as do pupils in state schools. A string of A grades may be an essential part of the package offered to parents by the school, but it is only one element.

What the stunningly good results of the independent sector show is how triumphantly the public schools have reinvented themselves over the past 30 years. Before that, the most exclusive schools used to contemptuously describe the big academic grammar schools - the likes of Manchester Grammar, Bristol Grammar or Bradford Grammar - as “sausage machines”, interested only in exam results. The aristocratic ideal of effortless superiority was assumed to be far more desirable than the swot toiling away at his books.

But an increasingly meritocratic job market which demands qualifications rather than family connections, forced them to change their tune. Around the same time, the “sausage machines” joined the private sector and began to compete with the traditional public schools. The enormous expansion of higher education meant that a university degree became the norm for the middle classes - and for girls in particular. The education market shifted, and the private schools changed to meet it. Now, top exam passes are crucial - and boy, do they make their pupils work for them. As the head of a prep school once told me: “What our parents demand is well-above average results for very average children”. In order not to fall behind in the race, he said, it was essential that every child in his school was reading at the age of four.

Of course not all these children are from super-privileged backgrounds. Private schools frequently point out that some of their parents are “first-time buyers” - but they’re not only buying exam results either. As a director of education once said to me: “It’s no good telling parents that school A, which has a dodgy catchment area, is actually achieving more than school B, because school B is full of middle-class children who are easy to teach. Your average parent will instantly think ‘Well I want my child in school B, with all those easy-to-teach middle-class children’.” Private-school parents decamp en masse to the state system because it’s better value for money? I don’t think so.