I'm old. Old enough to remember befriending the only black girl in the school, who got such a hard time that she left at the end of 1st year and went elsewhere. We met at a funeral over 40 years later and she remembered me being her friend. So I'm not stupid.
However it was 40 years ago. My children have gone to schools that were 'mixed' in any sense. They could give a shit about race, religion, sex, gender, sexual preference, none of it. I am not just imagining it, and my friends' children are the same. I was shocked to hear about a friend's DC who is going out with a DC of the opposite religion and gets a hard time from pupils in the friend's school!
You will find the well off from in and around Belfast etc will have holiday homes in Portrush and Portstewart, and they are student central. So I don't think what you are saying would hold true now.
I had one girl I'd been to school with take me to a Paisley service once (fuck me, that was scarey, I was sitting on my bloody hands so one didn't go up involuntarily when he thundered about whether we wanted to be saved!!!) I was 18 and green as the grass but when I didn't participate in the "silent collection", I thankfully wasn't asked back. Don't think I saw classmate again either! Nobody else has ever tried to convert me - other than Jehovah's witness and they get chased!
I don't think you spend enough time here to know how much it's changed. Not all of it. Don't think that ever will, because there are parents who had it bred into them now breeding it into their kids. But it is so, so much better!