Like yours, DD came from a state primary and I have had to remind her a few times over the years that her primary school was a much better reflection of the spread of ability in the real world!
DD was also much better at English and other things. And we have to remember that being 'bad' at Maths in St Paul's terms probably equates to getting an 8 or maybe a 7 at GCSE rather than a 9, though actually plenty of the lower sets get 9s too. These are excellent grades that the majority of people would be delighted with. And then she can give it up anyway and do something she likes better for A Level!