She has spent much of her time since regretting her behaviour in the early 1970s. So when Kenneth Baker, her Education Secretary, said during the 1987 general election campaign that the introduction of the new grant-maintained schools did not mean a return to selection, she moved quickly to contradict him.
Hijacking a morning press conference intended to trumpet the Tories' economic success, the then Prime Minister said it would be up to the schools to choose the pupils they admitted; the Government would not dictate to them how it would be done.
Warming to her theme during a later radio phone-in programme, Mrs Thatcher backed the idea of written tests for entry and expressed warm support for grammar schools, particularly in large cities. "I think", she said, "that they give chances which are not always given by comprehensive schools and, if there are proposals to set up new grammar schools, either from local education authorities or from teachers and parents, that too would be considered. "
Mrs Thatcher than boarded her "battlebus" and did not speak toher education secretary for the rest of the campaign. Mr Baker was left holding the fragments of his education policy and the voters were left confused.
The suspicion that the introduction of grant-maintained schools heralded the return of selection by the back door was reinforced. Not for the last time, a split between a radical prime minister and a pragmatic education secretary had become painfully public.
Kenneth Baker gave an assurance that no proposals for change in character would be entertained by the Government within five years of a school becoming grant-maintained. But no such clause appeared in the Bill and his impatient successor, Kenneth Clarke, removed the restriction and told a fringe meeting at the Conservative party conference in 1991 that he was expecting applications from GM schools to become grammar schools. "You only have to look at the parental demand for selection," he added.
When Mr Clarke repeated his views shortly before the 1992 general election, he let loose a storm of criticism. The Times asked: "Has Kenneth Clarke just cost the Tories not this election but the next one?" The 11-plus was the cross to which the Tory party was nailed at the 1964 general election, it said, adding: "If Mr Clarke has read his history, he is taking a fearsome risk I When the rejection notes start falling on the doormats of Britain's 11-year-olds, a dreadful spectre will arise to haunt the Tories."
Mr Clarke's successor, John Patten, was also keen to encourage selection. He issued a circular in 1993 permitting secondary schools to select up to 10 per cent of their pupils on the basis of ability or aptitude in music, art, drama and sport without having to ask his permission. A year later, he was encouraging Conservative councillors to present him with applications for grammar schools or the introduction of selective streams in comprehensives.
But schools and local authorities were reluctant to introduce more selection, opting out slowed to a trickle and the issue became submerged beneath the wave of educational protest over the national curriculum testing regime that eventually cost Mr Patten his job.
His successor, the pragmatic Gillian Shephard, might well have been happy to leave the issue of selection on the back burner. An educational insider married to a former comprehensive school head and a native of Norfolk, where choice is often restricted to one school, she is naturally more concerned with what goes on in schools than with changing their structure.
She was given the job to smooth the ruffled feathers of the teachers and she succeeded brilliantly. But Prime Minister John Major, egged on by his right-wing policy unit at Number Ten, felt that soothing the educational establishment was not enough in the run-up to a general election. He wanted a distinctively Conservative education policy - and one that, as the furore over Labour MP Harriet Harman's choice of a grammar school for her son was to show, would cause maximum embarrassment to Labour. The obvious answer was to boost selection.
So the Prime Minister started setting the education agenda, to the amazement and growing fury of his Education Secretary. In a speech in Birmingham last September to heads from grant-maintained schools, he announced moves to give GM schools more freedom over their admissions policies.
By the time the draft circular emerged in January, this had become a plan to let all schools select up to 15 per cent of their pupils on general ability. Then a White Paper was mooted that would go much further towards allowing schools to set their own admission policies.
It was during skirmishes between Downing Street and Mrs Shephard over the contents of the White Paper that the expression, "a grammar school in every town", first became associated with the Prime Minister. Not that he ever said it himself, but his spin-doctors made sure that the phrase found its way into print, along with suggestions that the new grammar schools would be initiated by parents, backed by private finance and perhaps located in the vacated premises of failed schools.
What Mr Major himself said, at a Conservative Central Council meeting at Harrogate in April was: "I believe choice and selection do have a place in education. And so do you. Not the old 11-plus - that's gone - but more selection in schools. And if parents want them, that could mean more schools like the old grammar schools."
And what the White Paper actually said, when it emerged in June, was that GM schools should be free to select up to half their pupils, and council schools up to one-fifth (but only with the permission of their LEA). The Funding Agency for Schools would be able to open new GM grammar schools.
There was no suggestion of a compulsory return to a selective system - and no enthusiasm on the part of the GM sector or the local authorities to bring it about.