John Rentoul: Whether he meant it or not, Cameron has just found his Clause IV moment

I didn't do it on purpose, seemed to be what David Willetts, the Conservative education spokesman, was saying. He had given a speech in which he repudiated grammar schools. The reaction from the Conservative Party and press was immediate, vitriolic and extraordinary. Even before the speech Matthew d'Ancona, the editor of The Spectator, had published a thundering denunciation.

On Thursday, Willetts was summoned before the 1922 Committee, and put in the stocks in front of the angry mob of Conservative MPs. David Cameron's advisers told journalists that they had not expected the reaction, and that Willetts had not said anything new - or indeed anything that Cameron himself had not said.

One of Cameron's friends told me that it was not intended to be a "Clause IV" moment, a symbolic confrontation with the orthodox sentiment of his party, but that it had "worked a treat".

I wonder. Willetts' speech was, unusually, trailed by the Conservative press office the day before. Cameron is media-literate enough to know that just because something has been said before does not stop it being "news" if it is on a "hot button" subject and is presented in an emphatic way.

And Willetts was recklessly emphatic. He said that the Conservative Party had to change. "We must break free from the belief that academic selection is any longer the way to transform the life chances of bright, poor kids." The hairs on the back of the neck of any Conservative would have stood up at that. "This is a widespread belief," Willetts went on - at this point blood rushed into right-wing temples - "but we just have to recognise that there is overwhelming evidence that such academic selection entrenches advantage, it does not spread it."

Cue explosion. I do not think I have heard a Labour minister say: "Academic selection entrenches advantage." But here was a Conservative shadow minister saying it.

This is no mere rhetorical cross-dressing. Generally, we know what Cameron is up to, and it makes some Conservatives a little uncomfortable. He has praised gay marriage. He has urged us to "show a lot more love" to hoodies. Last week, George Osborne, his shadow chancellor, condemned the fact that, under Labour "inequality is rising". (Actually, last week's figures showed the gap between rich and poor has narrowed under Labour, despite a slight widening in the most recent year; the point is that Osborne sounded like Polly Toynbee.) These are all the necessary compromises of pragmatic, One Nation Conservatism. The party will tolerate them while the opinion polls are favourable.

But academic selection is different. It is a supercharged issue for both parties. It drove the Labour Party to distraction when the Blairs sent their children to a religiously and socially selective school, and when Harriet Harman sent her son to a grammar school. The issue of selection provided much of the voltage with which Blair administered what he called "electric shock therapy" to his party. Now Cameron is using the same issue, from the other side, to do the same to his party. And poor David Willetts bore the brunt of the backlash last week.

Just as in the Labour Party, passions among Conservatives are in inverse proportion to the grasp of reality. All Tories know at some level in their deep unconscious that Margaret Thatcher as Secretary of State for Education closed more grammar schools than any other minister. They know, too, that they were in power for 18 years and did nothing to bring back the 11-plus. Yet they cling to the fiction that a future Tory administration would go back to the 1950s. This is based on the delusion - the "widespread belief" of which Willetts spoke - that selective schools are the engine of social mobility. It was hardly true in the 1950s; it is not true now, and it would not be true if the principle of selection were extended. Yet unholy myths are perpetuated by a conspiracy of Labour left and Tory right. The most important is that social mobility has declined under Labour.

It has not. Social mobility, like inequality, is complicated. The Blair Government's record on both is much better than the popular misconception. Tax credits, benefits changes, schools reforms and the expansion of higher education have improved life chances for many of the poor.

But people want to believe that bright children at the bottom of the heap have less chance than ever before. The figures on which this vast sky-castle of prejudice is built come from one study of people born in 1958 and in 1970. Those in the later cohort were more likely to end up in the same social class as their parents. It may tell us something about the grammar schools of long ago, although the manual working class was larger in 1958: other studies suggest a pattern of "trendless fluctuation" in measures of social mobility. But the life history of someone who was 27 when Blair came to power tells us nothing about social mobility in the past decade.

The other piece of evidence sometimes cited is the fact that test scores for poor pupils have improved more slowly than those for the better-off since 1997. Yet the definition of poor is those eligible for free school meals, whose numbers have declined by one-fifth (thanks to Labour's drive against poverty), so the figures are not comparing like with like. In fact, if you compare whole schools, the ones in deprived areas improved faster than those in richer neighbourhoods.

Willetts is right. Grammar schools may have provided a ladder out of the working class for a tiny minority in the 1950s, but secondary moderns were a barrier to social mobility for the majority. Over time, as schools became more socially polarised by house prices, selective exams became even less of a ladder and more of a way of entrenching advantage.

Whether Cameron meant this as his "Clause IV" moment or not, he has found it. The significance of Willetts' beating-up last week is that he and Cameron held their ground. This is a real test for Cameron, and he seems to be passing it. He pointed out on Friday that selection was abolished because it had become "deeply unpopular with parents" and he accused his critics of "splashing around in the shallow end of the education debate".

By promising to expand the academies programme instead of pie-in-the-sky grammar schools, Cameron is doing something superficially unpopular with the public, fantastically unpopular with his own party, but absolutely right.

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