Well you see again my experiences differ from assumptions on this thread.
DS's school: London hothouse, very selective academically. DC who were Jewish, Muslim, Hindi, Catholic, CoE and probably a plethora of others. Nationalities were: German, French, American, Australian, Chinese, Korean, Iranian, Russian, Indian, Italian, South African, Swedish, etc., there were also a few boys who lived in social housing on scholarships.
DD's school: not for the grammar school quotient and a very mixed ability but more homogenous and has been accused of being full of rich girls who are a bit dim and from the gin and jag belt. However, they all mix together and tumble along.
DD's comp: was a school of two cities and operated a bit like apartheid. The mc girls lived in one world and the others in another. The twain simply didn't meet and it was a very strange environment indeed. Exclusively unhealthy and different groups were ridiculed and excluded in a myriad of ways.