@Araminta1003 I can only speak for ds' school which is small (550 pupils) and very unfashionable. Pupil cohort is army, local professional, local farming families. There's no 'pre-chosen group' just a catchment area of about 25 miles radius plus a very small number of international pupils. No super-rich that I am aware of. About 10% SEN. Less money than you might think. We have a PTA, bake sales and fundraisers just the same as any state school. We've just paid for a new filtration system for the (outdoor) pool.
Standards of uniform and behaviour are focused on during pre-teen years. Smoking/vaping/phones are banned absolutely. Pastoral care and anti-bullying policies are very good. Parents and DCs sign behaviour contracts. The parents support the teaching staff. No in-term holidays.
By the time DCs hit teens, they know what is expected of them. Any sign of problems receives attention immediately. Violence/aggression/serious bullying with a three strikes and you're out policy, no exceptions. Individuals are not allowed to harm the life chances of others.
In 5 years my ds has been on the receiving end of a bullying incident once. The school called me to let me know what had happened before DS got home. They explained the process to deal with the bully. I didn't have to chase them.
Generally it is a happy place. Kids know what is expected of them. They know where to go for help. They feel safe. It's very straightforward.
I think a lot of people think of Eton or Harrow but it's nothing like that.