I was thinking about unconscious bias the other day. I can clearly point to the factor in my life that made me unconsciously biased against ethnic minorities and that was going to a school with loads of the human detritus. So much for the benefits of exposing children to diversity.
But by your own admission you went to a very good school that needed entrance exams. I suspect the ethnics you mixed with were of a higher standard than the ones I mixed with (i.e. middle class and/or with pushy, aspirational parents). And that may be why you came away feeling all warm and fuzzy about them and why I never wanted to see one ever again.