FWIW, I am a teacher in a largly mono-ethnic (Afro-Caribbean)school and I do understand Sails' concern about her child being obviously "different".
It's hard, being the only "different" looking kid. This is just as true for Sails' DC being the only white kid in a largly Aisian school as it is being the only black kid in a largly white school.
Lessons are geared towards whatever the predominant culture in the classroom is, and if it's not your culture it can be isolating.
It's hard to be anonymous. Everyone knows your face and your name; you can't get away with anything.
It's really important to teenagers to fit in. To have a shared culture (not necessarily an ethnic one) and identity with their peers.
I'm not in any way saying that this means a white kid can't have Asian friends and have lots of common ground with them. Nor an I saying that a white kid won't be happy or successful in a predominantly Asian school. Just that it is clearly a bit easier to be in a multi-ethic environment than to be the odd one out in a mono-ethic one.
And to say things like it shouldn't matter and kids don't see colour is nonsense. They do. Not necessarily because they're racist, but because it is part of being a teenager to identify those who are different (ginger, short, whatever). This can be good-natured harmless banter or it can be worse than that. This happens far less in a multi-ethic environment than a mono-ethnic environment, because differences and are less marked.