Almost any subject, from the structure of dust to the epigraphy of Sumeria, is interesting if only you are prepared to look at it closely enough.

An exception, however, might be made for the sociology of education, an intrinsically dull subject which deflects attention from fascinating topics (maths, history, literature, Greek, physics) to the boring subject of the social background of pupils.

That was in evidence in responses to a Sunday Herald report which disclosed that some Scottish private schools receive an 80% reduction in their rates bill because of their charitable status, while state schools pay full non-domestic rates (NDR). A lot of politicians and educationalists find this "inequitable", indeed "iniquitous".

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So it may seem, until one considers a few points. First, the NDR for state schools is paid by local authorities, who also collect it and are its ultimate beneficiaries. Schools with charitable status paying the reduced rate are thus paying more than local authority schools. If you charge yourself £1, collect £1 from yourself and then get that £1 given back to you, you gain less than if someone else gives you 20p. That is called arithmetic.

A separate point is whether charities should be given rebates at all. Not only are a surprising number of things charities (by some estimates, Scotland has the world's highest number per head of population), but mandatory rates rebates apply to many other things – pubs and petrol stations in rural areas, for example. The justification presumably springs from the social amenities that these businesses provide. That has nothing to do with whether they subsidise their services; pubs and petrol stations do not adjust what they charge their customers on the basis of their income, or redistribute the proceeds.

It's natural to think of charities as being exclusively about the distribution of money from the well-off to the needy, but many charitable foundations do nothing of the sort. The promotion of public health, amateur sport, religion, astronomical research, Gaelic, young farmers' clubs, pipe bands and almost anything else you can think of (these examples are from the official register) may qualify for charitable status.

If education in itself is an acceptable charitable aim, divorced from the question of who has access to it, then the obvious solution to any perceived inequality is for state schools to obtain the same status. The logic of the academy schools being introduced in England, giving control to schools rather than local education authorities, is a step towards that end.

The complaint against independent schools is, of course, not that they do not provide education, but that often they do it much better than state schools. Brian Boyd, emeritus professor of education at Strathclyde University, declared in response to yesterday's report that "closing the achievement gap" should be a priority. Unfortunately, his priority is not to ensure that children leave school having learned how to read and write. "The challenge for Scotland as a mature democracy," he says, "is to make equality its goal."

This Procrustean approach is presumably why Prof Boyd was at pains to deny that the teaching is any better in independent schools, pointing out that teachers in both are trained at the same universities in the same way. Alas, that is not true. Poor teachers (of whom, precisely because of the teacher training he identifies, there are still too many) are more likely to be rooted out in independent schools, since if they achieve poor results, no-one will want to send their children to them.

Under John Major's Government it was estimated that 15% of teachers were not up to the job, yet it is still almost unknown for failing teachers to be sacked from state schools.

Prof Boyd gave two reasons – frequently advanced by opponents of private education – why the state sector produces poorer results. The first is funding, which he says allows private schools to offer smaller classes and better facilities. But, as the increase in education spending under Tony Blair's Government demonstrated, this has little impact while the underlying educational philosophy of mixed-ability, comprehensive schooling is dominant. America has the highest education spending in the world, and poor results compared with countries such as Japan.

I have more sympathy with his second point, which is that able pupils, or at any rate those with supportive, ambitious parents, are "creamed off" by the private sector and contribute to the gap between independent and state schools. That is undoubtedly true, but it is a consequence of bad state schools, not good private ones.

In areas lucky enough to have good state schools, private schools ought not to be an attractive option. If anyone prefers them for perceived social cachet or some other frivolous reason, that is their folly, and their lookout. My own children attend state schools with better results than many private schools, but in most places that is not an option.

It is not difficult to see that these good state schools often result in effective economic selection by house price rather than fees. The shocking inequality here is not that some people pay for their children's education (as well as for the state system through their taxes), but that they feel they have to because the state provision is so poor.

Despite the inadequacies of some teachers, almost all the blame for this lies with politicians. A generation including the likes of Roy Jenkins, Tony Crosland, Shirley Williams and, though she later regretted it, Margaret Thatcher, were almost all privately schooled (some funded by scholarships or grants) or given a comparably solid education in academically ambitious state grammars. However, they pulled away the ladder which had given them the chances – and here I agree with Prof Boyd – all children ought to expect.

To offer those chances again, we must abandon the idiocies of grade inflation, dumbing down of curricula and subjects devoid of intellectual merit. The practices of the best independent schools – rigorous teaching, discipline, concentration on subjects of intrinsic value and the pursuit of elitism – should be adopted in state schools. The best way to challenge privilege in private schools is to make them unnecessary. Until state schools learn that lesson, they will remain a comprehensive failure.