What we think of as school quality is mostly the quality of the students. Richer districts have, on average, students of high SES with parents that are more wi

What we think of as school quality is mostly the quality of the students. Richer districts have, on average, students of high SES with parents that are more willing & able to help their kids succeed in school. Poorer districts have, on average, lower SES kids with terrible living situations and parents. Having disruptive kids also makes educating them much more expensive. Discipline issues chew up a lot of resources and disrupt learning for other students. So you have to spend more to provide equal quality instruction. And, of course, just because they spend more money doesn't mean they spend it on things that help educating students.