I went to private school in the 80s/90s. An all-girls from 11-16 and a mixed one from 16-18.
They were very different schools, the all-girls one was pretty mediocre tbh. I think I'd have got just as good an education at the grammar my mum wanted me to go to (my dad wanted me to go private, so I did). The mixed one I went to for sixth form was great though - such cool, switched on teachers, all highly educated, really inspiring people. I loved that school.
I think the main advantages are probably small class sizes and just the expectation that everyone would go on to university. Hardly anyone didn't. We were educated to believe that we could and would achieve and that hard work would get us through - and in most cases it did.
Which subjects were taught? All the standard English, maths, 3 sciences, hist, geog, French, German, Spanish, Latin, Greek, theatre studies, CDT, business studies, economics, art, ceramics, art history (and probably other things I've forgotten).
What sort of school trips were you offered? Russia, exchanges to whatever country's language you were learning, ski trips, field trips for geog, art trip to Greece ... can't really remember tbh!
Did you do much in the way of extra-curricular activities? I was in lots of plays and I sang in the choir.
Have your peers gone on to do great things (or marry well)? Some and some. The most famous guy I know I was at uni with, but I'd say most of the people I was at school with have done well, got professional jobs, own their homes, some privately educate/some not, all are pretty solidly middle class still.