to put it into context, my son's high achieving state school would have been absolutely delighted to have your daughter in Sixth Form.
My son on the other hand only just scraped into the same Sixth Form with a handful of B's and mostly Cs. I would have been so happy if he had had your daughter's excellent results.
This school still sends lots of pupils to Russell Group Unis, including Oxbridge.
Let go of your anger, shake your head and think..it's their loss not mine or my daughter's. She will do well at her new school (which presumably she is in now, studying for A levels) and you have learnt a lesson about buying into the spiel that top performing schools give about A level results. Top performing schools SELECT ruthlessly that is why they keep their stats.
We have a school near us (state) that does similar, throws out anyone performing under a certain benchmark, although by all accounts they would still do well at A Level. And then people say..they (the school) get excellent results - it must be a better school.
A school is like a family, it is heartbreaking to lose that "family" on such a small margin, but if your "family" is dysfunctional, it might be better to cultivate distance, or accept that you are going your separate way. It is worse when you have paid in, and participated fully, but nevertheless, along the way there will be many of these "betrayals" at uni, at home, at work, best to dust oneself off and move on, and be confident in what has been achieved, not lost.