So it isn't a creaming off of the best, and it isn't based upon selection at any particular age.
Except that it is just that. A school has to take children from more or less a given age, in order to prepare them for the all-important exams (the results of which, after all, are the means by which the pupils are able to show how talented or gifted they are to future employers/financial backers). Otherwise, we just let the cream rise of their own accord, as per folk like Richard Branson and Alan Sugar.
I'm not saying that music wouldn't be taught in ordinary schools Andy! I would expect comprehensives to provide great education to GCSE level in music. So ALL schools would prepare children for their exams and ALL schools would teach the core syllabus. But if you had a child prodigy cellist or violinist, the music teacher at the comprehensive would probably not suffice, a child who showed fantastic aptitude in such an instrument might move to a specialist school at the stage they were identified as such. They would still be able to study for the same exams that they were studing for at the comprehensive, though, of course, they might have to drop non-core subjects. They'd probably fly through their GCSE music exam, and also take some high graded examinations in the Cello or Violin (which they can take at any time they're ready for it).
If the system was the same now as it was 30 or 40 years ago, this would be a valid comment, Martin: but it isn't. Not only is the exam system completely different, but the social and technological context in which the education takes place is completely different.
You're right that much has changed. Remember I was making my argument against the lament that Grammar Schooling had been abolished and that the current system was poorer than the old Grammar Schools and Secondary moderns. If it's impossible to make comparisons, then it is equally impossible for the authors of the supposed articles

you were referring to. So you shoot your own argument down as usual.