I have a confession. I once nearly lost all my friends: this column runs the same risk. It was 1989, and the latest fad was grant-maintained schools, so I wrote a pamphlet called Every School a Private School, which earned a front-page headline in The Observer: “Labour adviser proposes all schools should be private”. Among my friends, “How could you?” was the usual response.

I was reminded of my ill-fated pamphlet not by the proliferation of academies and free schools - although about that it was alarmingly prescient - but by the grammar school debate.

The evidence-based arguments against this extraordinary proposal from Theresa May are now well-rehearsed: where else in the developed world is there a successful education system based on selection? Can selection at 11 ever be reliable or fair? The objections are legion.

Some think the grammar debate is just a distraction to divert attention from the formidable Brexit challenge, not to mention the funding of the NHS and other public services, including schools.

But taking Ms May’s proposal at face-value, she clearly thinks grammar schools will address the deeply embedded problem of social mobility. Like most people, however, I am puzzled by that assertion.

Where is the evidence from Kent or Lincolnshire that the poor do disproportionately well? In fact it’s the reverse. And she could test her ideas of reformed entry arrangements on existing grammars before embarking on expansion.

A ‘golden share’

So how does my long-forgotten embarrassment help Ms May? In the pamphlet, I argued that it wasn’t control of the curriculum or the budget that was the state’s main task: it was to secure fair admission arrangements. I suggested that, in addition, the government, through local authorities, should claim a “golden share” in all independent schools that benefited from charitable status, with the right to nominate 5 per cent of their entry.

Ms May once said that, had she come from a rich family, she too would have been pleased to attend an independent rather than Holton Park Girls’ Grammar School - which became the highly successful Wheatley Park comprehensive school.

One can understand the temptation: prestigious independent schools have hugely impressive facilities and their spending per pupil is high. The extra staff ensure more personal attention and more opportunities. The least they could do, then, is to welcome, in addition to the offspring of those at the top of our unequal society, those who come from the most disadvantaged families.

I am sure that local authorities, advised by their state schools, could identify those children whose lives would be blighted without such an opportunity.

Step forward Eton, Roedean, Winchester, Cheltenham Ladies’, Marlborough, Wycombe Abbey, Dulwich and many others. What will you do for your country? A positive response from you - not more grammars turning neighbouring comprehensives into secondary moderns - is what Ms May really needs as her educational social mobility policy. After all, she would have liked to attend one herself.

Sir Tim Brighouse is a former schools commissioner for London