The thing is, most people in most places are pretty decent and there are a few unpleasant ones everywhere.
I think there is a difference between being on the breadline and going to a top fee laying school, and being fairly middle class and on a bursary or discounted place because your parents work there.
Some children come from backgrounds of extreme disadvantage. For some, no-one has worked for generations and there is inadequate food and heating and clothing. They might have parents with addictions and serious mental health issues. Their lives include numerous difficulties and things which other children in all kinds of schools would struggle to relate to and which make them feel isolated. They will probably feel isolated in most schools and perhaps even moreso in a top fee paying school, or possibly no moreso.
Lots of children on bursaries and who have small scholarships or discounts because their parents work there or are clergy etc come from pretty middle class backgrounds. Their parents might not be rich but often they are educated and have had access to a range of activities. This might not include hugely expensive things like horse riding or skiing or globetrotting, but they might well have been involved in sport or music or all kinds of other activities. They probably have holidays and do t struggle for enough food or heating.
So what are the potential issues of going to a school where most people are very affluent? It can be the case that some children are snooty or snobby or very interested in money and wealth. It's usually the case that they are like this if their parents are. There are children and parents like this in every school, it's just the degrees if wealth that can vary but the attitude can be there. Most children aren't that bothered by them or impacted by them......unless they already have some kind of inferiority over wealth complex, which is to do with their own alrents attitude.
In terms of activities, lots of students don't go on the various long haul trips abroad. It's totally the norm. Many middle class children on bursaries or those of teachers have enough money to pay for some selected trips and activities and that's fine. Children and parents who feel aggrieved or hurt by not being able to do all the expensive activities could come from those on discounted places or those who are actually very well off but just not as well off as the richest....and who care about that stuff.
In the end, I really think it comes down to parental attitude. I have a friend whose son goes to a top school on a discounted place. My friend spends all her time talking about house prices and salaries and what she has compared to the other families. Her son has heard it all since he was very small. Both he and her have a bit of a complex abiut living in a smaller house and having less money than others. They both think about it a lot. I have another friend too whose child goes to the same school also on a discounted place, who is from a similar financial background...comfortable but not especially well off. They never talk about money or what other people have or what they have. I honestly think it isn't a big issue for them at all...and that's the adults and the child. It might be partly explained by the fact the second parents are highly educated and although not very well off, are academically and socially confident - they absolutely have no sense of feeling inferior. They will chat to anyone and be happy to do so.
So this thing about how parents feel matters a lot. If as a parent you've never ever mixed with the well-off and realised they are just people, you can decide they must be very different and feel inferior and inadequate. If you are confident in yourself as a person...your personality, your abilities etc and you teach that to your child (and it is by osmosis and how you live and who you are) then your child feels this too.
When people post on MN with this kind of Q and worries about not fitting in etc, I just feel a bit worried for some of them, because they make me think of my first friend. If it's a big issue for you as a parent, it might well be for the child too.
Working in an independent school can be a good way to see the reality....that children really are children in the main, and that most parents will be pleasant and a few unpleasant in all schools. Peoples lifestyles might be a bit different and their expectations be a bit different, but in the end, pretty much al parents sending their children to these schools want their children to do well and be happy...and you have that in common. Get to know some people and see, that especially if you're basically in the middle class, you can fit in very easily, unless you've got a chip on your shoulder.