I've read this thread and have a few points I'd like to add.
The first is sometimes people choose the less academic well thought of school because a child fails to get into the more 'prestigious'/academic school thinking Cranleigh rather than Charterhouse etc.
LeQueen categories of private school parent were interesting. There is another group that are the one time middle earning, 'John Lewis' types. Or at least their families were when they were growing up. They are highly intelligent and often grammar school pupils that progressed to Oxbridge or similar. Eventually through promotion and city jobs move to the huge pile with outbuildings etc and most take to be 'old money' - they have deliberately avoided anything remotely new build etc. They aspire to the elite public schools for their children sometimes because they felt disadvantaged by not having that status themselves on the way up. Not that you can generalise or any of it matters particularly.
Lastly there is a huge amount on this thread about IQ. I believe, as many of you will know by now, in the concept of 'learnable intelligence' what one can learn most can learn in the right environment etc. There are some very old fashioned ideas about IQ bandied about here.
I am not saying there are not genetic differences in IQ, but the most important thing is that everyone has a wide envelope of variation around that 'base point' that depends on experience, encouragement and self-belief.
It's in part the belief that IQ is hard wired that has caused our school to set so rigidly so early on. Yes, for a general 'sump' of IQ that will be the same for both literacy and maths so no distinction is made.
The views bandied about here about the fixed nature of IQ are ancient and long surpassed by better research. This was written as long ago as 1938 (Spens report) brought in to justify the introduction of the Grammar system:
Intellectual development appears to progress as if it were governed by a single central factor, usually known as 'general intelligence'..It's possible at a very early age to predict with accuracy the ultimate level of a child's intellectual powers'
Intelligence isn't unitary it is the sum of all of your habits of mind. The habits of mind are many and various and most of them can be cultivated. All this talk of IQ tests by Xenia etc and how she enjoys talking to people that have IQs above 130 etc. IQ tests are simply logic tests (that you can get better at by practice).