I went to 2 schools that were very mixed, at least 50% Asian and various other backgrounds too, and found it great.
However I then went on to a 6th form which had an even bigger percentage of Asians, and here I sometimes did feel it was a disadvantage - not because of any kind of racism, else I would not have been happy at the other schools and had friends of all backgrounds. It was more that a lot of the Asian students socialised only with each other and would sit in the common room chatting in Urdu, Punjabi etcetera (even though they were fluent in English too), and I often felt excluded.
So although I was happy to go to mixed schools, I was glad afterwards that I had not gone to the other local junior school, which was about 95% Asian at the time, as I think I would have felt very left out even though it is apparently a very good school.
I also think I would actually have learnt LESS about other cultures by going to that school, as at my schools we also mixed with black/white/Chinese/mixed race kids as well as Sikhs, Hindus and Muslims, so we learnt about pretty much every religion and culture, from Hanuman and the ten-headed demon Ravana to nativity plays and Chinese New Year!
So I can understand the OP's point of view, and don't necessarily think there's anything racist about it. And as she said, the school she prefers is also very mixed anyway.