I must say the last thing on my mind when looking for a school is the racial mix of the school.
That is because I went to international schools abroad (ok, my friends were predominantly rich kids with middle class or upper class parents with a high level of education, but my argument still holds) when I was school age and I can't think of anything better for my son than mixing with children from other countries and religions when he is growing up, just as I did. In fact I think making friends with other ethnic groups and religions is an importnat part of his education, just as it was for me.
If the school has a good reputationa and good exam reults, there is absolutely no way one could say that multiculturalism in that school isn't working. It must be working if the children get on well enough to enjoy themselves and to achieve good results.
The local authority primary school I have picked for my son has around 30% from a non-white English background. It is multi-faith and at least 10 different languages are spoken. I honestly would not send my son to a school that is 100% English, white and middleclass.
I would have found it very dull as a kid if I had been to a school like that as most of my friends up to the age of 27 when I finished my Masters were foreign. I want my son to have the same kind of education in a multi-cultural environment. I think he will become a very boring individual indeed if all his friends are white, English and middleclass and don't speak a single foreign language. That's my view any way based on my own experience.
Oh, and my view of multiculturalism is that "it takes two to tango". It makes me laugh when people who make absiolutely no attempt whatsover to get to know their foreign neighbours then say "oh, well, it doesn't work". It would work better if us the natives made more of an effort too.