Garvan Walshe is a former national and international security policy adviser to the Conservative Party.

Controllers at Holborn tube station in London have made an surprising discovery: that if everyone is made to stand on the escalators, instead of having people stand on the right and walk on the left, more people can be got through the escalators (because there are usually big gaps between people walking).

There’s a word for this kind of thinking: socialism. It deprives people of freedom – in this case, to choose to walk up if they want to get somewhere more quickly, or to stand if they can’t or don’t want to or can’t. On Holborn’s socialist escalators, everyone stands.

In the narrow world of escalator operation, this achieves its goal: putting more people on the escalators, at a cost of nobody being able to get out quickly. In this, it’s like comprehensive education: sure, nobody is left to rot in a secondary modern, but nor is anyone stretched like they are at a grammar school. Actual schools, of course, depart from this caricature – yet this is, or should be, a basic philosophical difference between the parties: does the school system exist to equalise opportunity, or maximise it?

By hinting she is open to new grammar schools, Theresa May has allowed the debate about school selection, stifled since David Cameron ruled out building new ones, to revive. To have this debate properly, some myths have to be dispelled.

First, all schools that are oversubscribed can’t avoid selecting pupils, and the school admissions code prescribes in considerable detail how it is to be done. The academy movement’s soundbite that “we don’t want schools to select pupils, we want pupils to select schools” doesn’t work without significant over-capacity, and is plainly impractical in rural areas where there is usually only one secondary school around.

Nor should the grammar school system of the 1950s, designed for a very different society and economy, simply be replicated now.  To take a few obvious flaws: when 50 per cent of a generation is supposed to go to university, selecting just ten per cent for an academic schooling doesn’t make sense. We now know that pupils develop at different rates, and a one-off exam at age eleven could be replaced by a series of tests, as it is in the Netherlands. Nor need there be a two-way division into academic and technical school systems. The curriculum is far more varied than it used to be, so there is no reason for all schools to select on the same criteria.

Assuming that we are not to recreate a system in which academic tests define who gets into which schools, this shouldn’t none the less be almost the only factor excluded.  It should still be possible to make it part of what is now. rightly. a much more complex admissions process (because schools have taken on a lot of the work that used to be done by extended families and stay at home mothers).

Indeed, schools are more than just exam factories; they’re social institutions. Parents naturally worry about getting the best grades and opportunities of their children, but they are also anxious, often with good reason, that other parents won’t nearly care as much as they do, and will bring up their kids badly. They want to be sure their children will be safe from bullying and violence. They’re scared that they will fall in with the wrong crowd, and end up pregnant or in prison.

Even if, from the abstract perspective of a specialist in social policy or the parent who sends their children to an independent school, badly-behaved children from difficult backgrounds are more in need of the state’s attention, it is difficult to explain to other parents why their child should suffer as a result because simply they can’t afford the fees, or a house in leafy area. An element of academic selection provides both an incentive to play by the rules that will stand children in good stead in later life – and a means of escaping those who don’t.

The comprehensive monopoly on admissions has done so much to impede opportunity in the name of an equality that levels down. It’s time to bring it to an end.