WRITING about grammar schools always gives me a headache. I’ve just been listening to the education secretary on the radio and that only thickened the pain.

You see, Justine Greening is going to give a speech today about grammar schools. With distant memories of my own school days, I’d like to throw some chalk at the education secretary, or possibly even one of those heavy blackboard rubbers. We had a maths teacher who did that called, if  memory can be relied on, Mr Simms.

What Greening will say that Theresa May’s new grammars must give priority to “ordinary working class families” rather than just children from poorer families eligible for free school meals and so on.

I don’t have this from the education secretary first hand or anything, as we aren’t that close, but her speech has been trailed all over the place. Which is a bit like those overblown cinema trailers you get nowadays: by the time you’ve sat through the promo, there is hardly any need to bother with the film.

Anyway, the trailers for Grammar School: The Movie place great emphasis on that word ‘ordinary’. Politicians often use ‘ordinary’ as a catch-all label to smooth over the creases and to give the appearance of being fair-minded and reasonable.

Yet the seemingly plain-speaking words in Justine Greening’s trailer prove surprisingly slippery when you try to grasp them. ‘Ordinary’ in this carefully arranged context leaves out the very poor but draws in children more likely to live in Conservative constituencies rather than Labour-voting inner cities (that, of course, is assuming anywhere will be Labour-voting next time round).

The Government has massaged a few figures to come up with a definition for those “just about managing families” whose existence is such a political touchstone for Mrs Maybe. This is retrospective number crunching: think of a social concept, bang on about it for a while, then later attempt to come up with an official definition that justifies what you want to believe.

Now if you ask me, the government is cooking the books when it comes to grammar schools. There is little evidence that grammar schools improve social mobility; most education experts believe that grammar schools are not socially improving; and many teachers think the same, too.

Grammar schools are as much about ideology as anything else. The very words scratch an old Conservative itch and summon up the rosy glow of a caned hand. And the evidence suggests that they benefit the better off because they rely on entrance exams such as the 11-plus as a means of selecting pupils – and wealthier parents can afford to pay for private tuition.

But then I went to a grammar school and according to this government’s logic, that makes me one of those people who want to pull up the ladder behind them – a silly criticism that was made of Labour leader Jeremy Corbyn, another ex-grammar boy – after we have benefited from whatever it was grammar schools were meant to have given us.

The trouble is, it was all such a long time ago, and yet here we still are, trapped in the same old gluey arguments about grammar schools. You could just as easily make a perfectly good case for schools that accepted pupils from all backgrounds, mixed them up and benefited from the strength of a wide and deep social pool. Oh, you could even call them ‘comprehensives’ – after all, it’s just another word. Sadly, words carry a lot of baggage.