I was incredibly lucky with my school, more than I realised at the time or for a long time afterwards. This was in a Northern city in the 1970s.
Direct grant school - fee-paying girls' school but about 25% of the intake were on 100% scholarships from the LEA and probably a majority of the rest were on means-tested fees (subsidised by the LEA) which in some cases were almost nothing. Very academically selective. We had some girls who would certainly not have made it through the entrance exam if they hadn't attended the preparatory department for several years and others who wouldn't have got a scholarship if they hadn't gone into the junior school for the last year or two of primary. But mostly we were well above average.
Every single teacher had a degree in the subject she (almost all of them were female) was teaching, except the PE, domestic science, art and music staff who had all attended specialist colleges and had diplomas. Every girl in the school did English, Maths, Physics, Chemistry, Biology, French, Latin, History, Geography, RE, PE, Art, Music and Domestic Science in the first three years. We all had to do English, Maths, History, Geography, French and at least one science to O level. Almost everybody got 9 O levels at C or above, with the majority managing to get mostly As and Bs.
Out of about 85 in my year, maybe 10 left after O levels, usually to continue with some other form of education elsewhere. We got some very bright girls indeed transferring in for the sixth form. Out of the 85 or so who took A levels, I believe 10 went on to medical school, one to dental school, one to veterinary college and another 10 at least went on to do degrees in Maths, Physics, Astrophysics, Engineering or Chemistry. There would probably have been another 10 or so who went on to Biology or Biochemistry. There were about 10 girls who went on to Oxford or Cambridge (various subjects). Over 80% of the year group went on to university and almost all the rest went to either a polytechnic or a teacher training/art/PE college or to do professional training in nursing, physiotherapy etc etc.
I only grasped years later that by the standards of the time our results were phenomenal.