The regimentation and rote of your average maths classroom was obviously never designed for teaching the subject, yet is the main memory of the experience for most people, rather than what is supposedly being taught.
With stats like that, they REINFORCE class structure, not break it.
And do poor people want to pay for the education of the elites? Which is what current grammars effectivly do.
These days they are intended to reinforce class structure. You don't need a left-wing, if you don't have a poor working class...
The only reason for populist support for them at the moment(Daily Mail, Telegraph, UKIP, BNP and other populist parties all support them), is most people THINK their kids will get in although most won't(By definition). When they come realise
that, populist support will disappear again, and they will go out favour once again.
Pointing out that those who leave grammar school have high literacy and numeracy hardly demonstrates that it is a good way of teaching, when the intake for grammar schools is those with high levels of numeracy and literacy already.
If you want to look at a true education revolution look to Finland, which is about as far from the grammar school systems as you can get, but also has an astoundingly high level of educational excellence.
Meanwhile, Asian schools were cranking out kids who had actually understood the material. How 'bout that?