There was also a significant minoRity who were disruptive and unmanageable and they had a negative impact on learning and enjoyment. State schools have to admit girls like these and for budgetary reasons are reluctant to permanently exclude.
State schools with amazing reputations are always oversubscribed and keep waiting lists, so permanent exclusion of disruptive children will have no budgetary impact and a very positive effect on the remaining children. Budgetary reasons are an excuse for a weak Governing Body not supporting the Headteacher on demonstrably fair, sustained and decisive action to exclude permanently. The school will deservedly lose its "amazing reputation" if it does not act quickly to stamp out unacceptable behaviour.
As to OP's original question, I agree with many here that it depends on the individual schools and your child. Remember that your child will not always be 7/11/13, and competent teachers easily remember many thousands of children long after they no longer teach them. The new entrants taking you around on open days are a good indicator. Their parents have newly chosen that school, so that's a powerful source of information. Which schools they considered, why they are there, what their aspirations are, etc.
I have seen ability/attainment segregation at long established outstanding public, comp and grammar schools. The only difference, provided you feel the school is good enough for your child, is that when you pay lots of money, you assume they will bring out the best whatever set/stream/class your child ends up in. There are better and worse teachers in both sectors and what you don't (wish to) see you won't mind. Your child will signal clearly to you whether they are engaged and up to scratch, even if that is only in the school holidays. Again you can choose whether to take note.
It also helped us to remember that being stressed out doesn't change which school they end up in. It only makes life less pleasant for a while.