When educated and motivated parents pull their children out of public schools, they are taking motivated, capable students out of public school classrooms. They are taking out independent learners, peer role models (positive peer pressure is a powerful force), someone who might understand what the teacher is saying and be able to explain it to their classmates, an intelligent participant in class discussion. To me, the struggling learners are not so difficult to teach, but when you get a class where most of the students don't care about school, whose parents are happy with Ds, that's difficult.
Parents don't like their children being viewed as some kind of social resource to be used in improving the achievement of other children. They make schooling decisions based on how much their own children will learn and otherwise grow.
If a middle-income school district had ability grouping, subject acceleration, and grade acceleration, I would consider living there and sending my children to the public schools. Since all public schools in Massachusetts are heterogeneously grouped and there is no acceleration, we moved to the town within our commuting radius with the highest test scores, since that it is the closest we can get to ability grouping. The new town is, not coincidentally, more affluent and less diverse than the town we used to live in.
Don't try to use people's children as "role models" for the benefit of others. Their parents will not stand for it.
