Wow, that was my high school! But I'm going back 30 years since I started. Certainly didn't feel competitive back then, although it may just have gone right over my head! I joined from a primary school that would probably be classed as underperforming by today's standards, only 2 out of the year 'passed' the 11+, most of my friends went to Quarrendon and I was devasted not to be going with them!
Most of the intake seemed to be from surrounding village schools, there was quite a mixture of people, income brackets and ability. One of my friends lived in a pretty standard semi in Wendover, another had a huge house in the country with stables and owned a horse.
I certainly wasn't competitive, I was pretty standard academically and rubbish at sports etc. But I did pretty well and was never made to feel that I wasn't good enough - I was the third year of GCSEs, so grading expectations were a bit of unknown. Most went onto do A-levels, a few did leave to go to college at that point instead. There was an expectation that you would go onto university, and limited support if you wanted to do something vocational instead (my Maths teacher was visibly shocked when I said I wanted to be a Holiday Rep not go to Uni to study Maths!).
I didn't even get to look round the school, my Mum made the decision that was where I was going. I was a very shy child and she believed the single-sex education would help me come out of my shell. She was right, and I made some really great friends, but girls can be incredibly bitchy and if you got on the wrong side of the alpha girl you were in trouble. Not sure whether this would be tempered in a mixed school, or whether that's just teenagers for you.