THE first absolute requirement of any currency is that it should be consistent. Unless a pound coin is universally agreed to be worth the same as every other pound coin, no one can have confidence in any transaction.

Few transactions are more important to most families than the education of their children. Yet the debate about admissions to Britain’s top universities, so spectacularly inflated by the chief custodian of our monetary system, Gordon Brown, has been conducted throughout in terms of a currency - A-level grades - about which there is no such absolute confidence.

So far as I am aware, no serious contributor to the national controversy sparked off by the case of Laura Spence has questioned the assumption that three A grades at A-level represents, in effect, the pinnacle of academic achievement at school and should, therefore, be the passport to a course at Oxbridge or a Russell Group university. The criticism of those universities has rested on that assumption. Roughly 30 per cent of all candidates achieving three A grades come from independent schools, the argument goes, so any university accepting a higher proportion than that must be discriminating unfairly against state school applicants.

But, to put it bluntly, is every sixth-former’s 30-point haul at A-level worth the same? Every headteacher’s experience will suggest that it is not: that there are some subjects in which it is harder than others to achieve a top grade. University admissions tutors, too, will acknowledge privately that there are some subjects offered at A-level which they regard more highly than others. Fortunately, however, we don’t have to rely on the anecdotal evidence of heads or dons. There is objective evidence to hand.

During Lord Dearing’s time as the Government’s education guru, the School Curriculum and Assessment Authority (the then curriculum watchdog) commissioned a report from Professor Carol FitzGibbon, whose work on value-added in the sixth form has achieved official recognition. She found sharp differences in the relative difficulty of A-levels and that, in particular, students taking exams in physics, chemistry and maths faced a harder task than their contemporaries in getting similar grades.

When you start to examine the relative achievements of candidates from independent and state schools in different subjects, this finding becomes very relevant to the current debate.

Analysis of 1999 A-level data from the Deparment for Education and Employment shows that 22.7 per cent of all A-level examinations in schools were undertaken by independent school candidates (itself three times more than the oft-quoted 7 per cent of the whole school population in independent schools). A much higher proportion of all entries which achieved an A grade - 38 per cent - came from independent schools.

But look at the “hardest” subjects and the discrepancy is even more stark. In chemistry, 30 per cent of the school entries and 45 per cent of the A grades, in physics, 27 per cent of the entries and 42 per cent of the A grades, and in maths, 28 per cent of the entries and 40 per cent of the A grades, are achieved by independent school candidates.

And it doesn’t stop there. In economics - another of Professor Fitz-Gibbon’s “harder” A-levels - independent candidates accounted for 31 per cent of those from schools and 47 per cent of As. Modern languages were also rated as harder than average, which may be one reason why numbers of sixth-formers studying them have been declining. Here the independent sector’s contribution is even more marked: in French, for example, more than a third of all school A-level entries and 52 per cent of all As were from independent schools..

The debate the Chancellor started is a perfectly proper one. Nobody doubts that some young people under-achieve at A-level. There is no serious dispute that universities should take steps to identify young people who have been badly undermined by their circumstances or, in some cases, by their schools. But to pursue radically different admissions policies, on the basis of unfocused school A-level statistics (or worse, on the basis of postcodes) will simply replace one injustice with another, denying opportunities to talented young people on the grounds of decisions made by their parents.

As for independent schools, so far from taking an unfair proportion of places at the best universities, the evidence suggests that they are making a disproportionately large contribution to ensuring that the nation has a new generation of scientists, mathematicians and linguists.

The real debate should be about how to open up what independent schools have to offer to all sections of society, rather than trying to bully universities into adjusting their intakes according to specious, but politically correct, criteria.

Dr Philip Evans is the headmaster of Bedford School and co-chair of the

university committee of the Girls’ Schools Association and Headmasters’ and Headmistresses’ Conference. He is writing here in a personal capacity