Just what is the problem with grammar schools?

We find the current furore over grammar schools to be puzzling, difficult to understand.

Take this in the Observer. They note, correctly, that Upper Bavaria has an astonishingly low youth unemployment rate of only 3.4%. They credit, correctly, the education system:

Germany’s much-admired dual education system (in which apprentices are trained jointly by employers and at specialist vocational schools) has grown in Upper Bavaria, not because it was seen as the more responsible thing to do, but because companies, unable to “buy in” a fully trained-up workforce, often had to mould the workers they needed themselves.

That education system is dual in another manner too:

 The Gymnasium is designed to prepare pupils for higher education and finishes with the final examination Abitur, after grade 12, mostly year 13. The Realschule has a broader range of emphasis for intermediate pupils and finishes with the final examination Mittlere Reife, after grade 10; the Hauptschule prepares pupils for vocational education and finishes with the final examinationHauptschulabschluss, after grade 9 and the Realschulabschluss after grade 10.

Perhaps not dual then but there's a definite separation into academic and vocational streams - streams which go to different schools. And this is a much-admired system which leads to a 3.4% youth unemployment rate.

Yet mention grammars in Britain, the equivalent of those gymnasium (and common across much of central Europe too) and we are discussing the very devil, encouraging the destruction of all that is good and holy in our society.

We don't understand it. Our assumption is that it must have something to do with left wing politics - we find that's the usual explanation for beliefs entirely at variance with reality.