If you are going to have schools for specially gifted kids (which you agree is a logistical necessity) you have to select at some point which kids go to those schools and which do not. How is this compatible with your bald statement that selection at a given age is wrong?

Well firstly, just because you have schools for the exceptionally gifted it doesn't mean that selection for those schools must be done at any one age. But, secondly, to state the obvious, if one or two exceptionally gifted musicians are selected from a comprehensive school it doesn't mean that they are better than the pupils who are left, it simply means that they have a special gift in one particular area that, for obvious reasons, can be developed better with specialist tuition.  There will be other gifted children whose gift doesn't require such specialist teaching or equipment to properly develop, who will continue to be educated at the comprehensive, and there will be good all-rounders who will continue to be educated at the comprehensive school.  So it isn't a creaming off of the best, and it isn't based upon selection at any particular age.

Compare that with dividing the population into two, the top half and the bottom half, on the basis of an exam at the age of eleven and it is stating the bleedin' obvious that this is not, in any way, a similar thing.

This must be the place, second place in the human race,
down in the basement, now I know what she meant,
this is the hand that you never shook,
you never gave me the chance that I took.  Elvis Costello - Secondary Modern.

As regard to the rising pass marks in exams, yes there are other factors which affect the figures, but you cant have 24 years of rising pass marks without the education system having got better at its job.

I am not saying that the old system was better - simply that the comparison is much harder than you are suggesting, because so many of the parameters are completely different.

I'm glad you're not saying the old system was better, and I agree that there are factors which don't make comparison over time easy, but I think it would be very hard to argue that the system is worse than it was, and that you would find much more evidence to support the idea that it has improved over the years.