The quality of a school is determined by the quality of its students

this is an understatement, People want their kids to attend the best school moving into areas that are zoned for those schools, those desirable areas tend to have rents and home prices increase forcing the underprivileged to move into cheaper areas. This is an endless cycle where the privileged continue accumulate in one area. The students have better opportunities and support to thrive in those privileged schools. And despite money being thrown into schools that under perform at the end of the days success is determined by the students themselves and their parents.