It's perhaps worth to consider what does "quality children" mean in the context of a "good school" - and it could certainly be argued that one of the major criteria for parents moving in and choosing a school is to get their kids into a school where the other kids are of a comparable or higher socioeconomic status. The other things about the school are fixable by heavily involved parents, but you can't (easily) alter the peers which your kid has in school, only pick a school with "good" peers. Other things being equal, having your kid in a school with (other) poor kids puts him/her at a disadvantage, all kinds of poverty-correlated problems (e.g. parental neglect, parental drug abuse, nutrition problems, crime and imprisonment) drag down not only affected kids but also their classmates.