Pissfarterleech-
I don't choose to spend time with certain types of people and I suspect that's true across the board.
This is a point I wonder about too. The vast majority of adults choose to hang around with, get to know and develop ongoing relationships with, those who are similar to themselves in some way or another. Similar interests; similar levels of education (leading to similar cultural and conversational assumptions); similar income levels (allowing similar expectations about what to do together) etc. etc. I don't really know any adults who just go and sit in the middle of their town square, talk to whoever happens by, and tell themselves that that person now has to be their friend no matter how little they have in common with them or how uninteresting they find them.
So why do some of the same adults insist on the idealistic fantasy that this is what children should be forced to do?
Tendencies, interests and character traits grow and thrive by interaction with others with the same traits. Most of us want to nurture some traits or potential traits in our DCs and minimise others. Someone who is academic will grow in that way from being around other people who are; someone who is musical will go further being around other musicians with whom they can share certain assumptions and activities, and so on.
Now it's probably true, particularly in the posher public schools, that for some people this comes down purely to wanting to mix with the rich and well connected, and that's a very superficial way to raise your kids. But I honestly think for most people it isn't about that. I think most people who send their kids to private schools do so for the same reason we're sending ours to grammar - they want them to be around other kids from families who value and respect education, and not to feel pressured to suppress their natural appetite for learning just to fit in to a social ethos they (the parents) don't share, don't like and don't want for their kids.
Which is not to say you can't find this in some comprehensives. But unfortunately it's only some, and it depends where you live.