At some point, old certainties begin to crumble. Since the days of Thatcher and the triumph of "the market", parental choice has been taken as an unassailable given in educational politics.
After eight years of a Labour Westminster Government, one of Scotland's leading MPs said to me, with a resigned smile: "But there really isn't anything we can do about parental choice except make the poor schools better," (as if, somehow, the issue was the "poor" schools rather than the social stigma of the areas and the pupils they serve). He at least had a sound record in sending his children to their local school.
More recently, senior local officials made almost exactly the same remarks: "We have to live with parental choice. How best do we do that?"
Yet change is in the air. An aspect of one significant Thatcherite shibboleth is about to disappear. The Scottish Government has taken a tentative step forward in the field of housing by abolishing the tenant's right to buy in respect of newly built council houses. MSP Margo MacDonald has suggested that at a time when the sustainability and number of schools required is the focus of a very real debate, parental choice is also worthy of reconsideration.
The problem is, of course, most acute in Edinburgh. Some years ago a colleague of mine, a principal teacher in an Edinburgh secondary serving a peripheral area, achieved an assistant headship in Glasgow.
"There's only one difference," she said, "between the two schools. The catchment areas are similar, so is the social mix, but in Glasgow almost every local youngster attends the local school."
Edinburgh, alone in Scotland, has well over 20 per cent of its secondary school pupils in the private sector (don't tell me it's because Edinburgh's comprehensives are poorer schools than their equivalents in Glasgow, Aberdeen or Dundee).
The knock-on effect of this is that all schools in Edinburgh are perceived to be on a status ladder, with the status of the school being overwhelmingly set by the surrounding catchment area.
And because Edinburgh's social elite (and it's a larger one than elsewhere) buys its way into educational privilege, the next cohort on the social status ladder seeks to emulate that by achieving a second-grade version of that privilege via parental choice.
There are two ways round the problem of avoiding the magnet schools and their antithesis - the schools of last resort. One is the abolition of parental choice. The other is the drastic redrawing of catchment areas so that no school serves either an area of unmitigated poverty or one of overwhelming privilege.
Perhaps even those who favour an end to the market principle of school choice would be better served by starting with the redrawing of catchment areas. Margo MacDonald, however, should continue to challenge the parental choice paradigm. Education, after all, should be a right for all children, not a privilege to be gained by either parental wealth or ambition.
Alex Wood is head of Wester Hailes Education Centre in Edinburgh.