Sam Freedman is Executive Director at Teach First. From 2009-2013 he was a policy adviser to Michael Gove.

Grammar schools are great for the children that go to them. Unfortunately, they’re terrible for the vast majority of children that don’t. This is one of the most conclusive findings in education research (see the IFS summary here) and the Government, to their credit, acknowledge it in their Green Paper published on Monday. With some understatement, they admit that: “under the current model of grammar schools…there is…evidence that children who attend non-selective schools in selective areas may not fare as well academically – both compared to local selective schools and comprehensives in non-selective areas.”

Essentially, the Government argument is that because selective schools get good results it would be desirable to have more of them, as long as the negative effects on the rest of the population can be alleviated. Much of the Green Paper is thus taken up with trying to work out how to mitigate the ill-effects of its own proposals. It suggests that new selective schools might have to run their non-selective counterparts or, perhaps, they could take a quota of pupils on free school meals. But none of these suggestions deal with the fairly obvious problem that opening a selective school in an area automatically handicaps every other school – whoever it’s run by.

Government ministers keep telling us that adding selective schools to the mix will increase choice and diversity within the school system. But Conservatives of all people should understand that choice requires a functioning market and markets require fair competition. Selective schools destroy choice and competition because the odds are so rigged in their favour. By being able to choose their own customers, they inevitably make all the alternatives less attractive.

I have three young children. At the moment I can choose between three comprehensives for them. If one of those became selective, my choice would disappear. I’d have to enter them for the eleven-plus to try and get access to the school that has all the advantages of selection. And if, even after I’ve shelled out thousands of pounds for private tutors and gone through months of domestic stress, they didn’t make the cut I’d be left with no choice at all. The remaining comprehensives, which would become secondary moderns in all but name, are it, unless I could scrape enough money together to move or go private – which, of course, wouldn’t be an option for the vast majority of people. Rather than diversity, selection offers parents a gamble where the odds are on losing.

This is why parents are much less keen on new grammars than much of the media seem to realise. Last month YouGov put out a poll with three options: open new grammars; keep the existing ones; close all grammars. Opening new ones only got 38 per cent of the vote with 40 per ent preferring to keep the existing system or move to an entirely comprehensive one. But crucially older people were heavily in favour of new grammars. The 24-50 age group into which most parents fall were against by 44 per cent to 29 per cent. It’s easy to forget that the grammar system of the 50s and 60s was dismantled not because of a central Government decision, but because parents forced the hands of their local councils. Margaret Thatcher closed more grammars than any other Education Secretary not because she opposed them, but because even she couldn’t resist the clamour.

For all that the Government wish to have their cake and eat it, we have to accept that selective systems represent a trade-off between a small minority than benefit and large majority that are penalised. And that would be a trade-off that is not only profoundly unfair but also deeply damaging to our economy, which relies on an ever-increasing number of highly-skilled young people. With competitors in the Far East, Scandinavia and elsewhere increasingly moving to education systems in which all young people benefit from some kind of tertiary education – be it university or high-level apprenticeships – it makes no sense for us to focus in on maximising the chances of an already privileged minority.

Instead, we should look to our great educational success story – London – for inspiration. Over the past 20 years inner-London schools have gone from the stuff of tabloid scare stories to some of the best comprehensives in the world. The poorest children in Tower Hamlets and Westminster now do better than the average for all pupils in selective counties like Kent and Lincolnshire. Over the past few years I’ve met delegations from all over the world – from Belarus to Kenya – trying to understand how to copy the unique success of our capital. I’ve met no one interested in copying Medway. It would be perverse if we didn’t try to learn those lessons ourselves.

And one of the biggest lessons is that all children can benefit from an academic education well beyond the age of eleven. I would urge any supporters of selection who remained unmoved by this article to visit a school like King Solomon Academy in Westminster; Bethnal Green Academy in Tower Hamlets or Reach in Feltham. These schools, and many others like them, are proving that it is possible to give all children, regardless of background, an outstanding education.

It would show immense poverty of ambition as a country if all we could hope for was a good education for the minority. As a parent I don’t want a shot at a good school place, I want a guarantee and that is simply impossible in a system with selective schools.