Statesmom, these schools are not aimed at kids with merely 'not below average' IQs.
I wouldn't like to put a figure on which percentile they are selecting from, and it can be misleading, because for instance you can be have an IQ in the top 5%, say, but be 0.001% for Maths, and the latter is far more useful than a mere top 5%.
I have heard it said that Oxbridge is +2SDs, i.e. a 130 IQ, or the top 2%.
Selecting the top 1 or 2% and then expressing surprise when they do well is rather naive.
Westminster sends 1/2 to Oxbridge, as opposed to 'only' 1/3 at Eton. But that doesn't prove Westminster is better at this job in the slightest. A boy might actually have a better chance of making to Oxbridge from Eton than Westminster. The raw stats don't prove anything.
What I would say is that Eton has more money and much better facilities, and there's more to it than just obsessing about Oxbridge. In fact they aren't focused on Oxbridge at all in terms of their marketing. Whereas Westminster's incredible success in this area might well be self-selecting in terms of the parents who go there.
Even at Westminster, with perhaps the brainiest intake of all schools, who, based on IQ tests, should perhaps ALL be going to Oxbridge, still have 'half' failing'.
Finally, Oxford and Cambridge are certainly inferior to US colleges in terms of their funding and probably opportunities too, and it's a very arbitrary benchmark to define a school as the best in the world on that basis.
For someone calling yourself Statesmom, I think if you want to find 'the best school on Earth', you need to look over on the other side of the ponds. The likes of the Phillips Academys have a billion dollars floating around in their endowments, and there are plenty of schools in the US sending a similar % to the top elite US universities as Westminster does.