I doubt whether it's a question of race, as such. People want their kids to get a first-rate education, and if substantial numbers of other students in the classroom with them are less prepared or even less bright or can't speak the language well, it pulls the whole class down.
I don't know what you can do about this kind of problem. Regardless of the period, there have always been situations where some kids are born on the wrong side of the tracks. In the past kids would be streamed, or maybe some of them simply wouldn't go to school but would work on the farm starting at an early age.
It's impossible to have an excellent, demanding education that stretches kids to the limit and an education that treats all kids exactly alike, regardless of ability. In America you just have an obligation to give every kid a CHANCE to prove himself, or periodic chances to straighten themselves out and switch over to the faster track.
People tend to gravitate to people of their own kind. It is just the way things are. That is why you have pockets of all varieties of people living all over the place. First generation of whatever background tend to gravitate to each other. Nothing new. The superintendent was embarassed by the presence of so many Mexicans. It is just the concentration of labor force in the town.