As a former teacher in one of the poorest congressional districts in the country at the time (South Bronx, early 2000s), as well as the current parent of a kid

As a former teacher in one of the poorest congressional districts in the country at the time (South Bronx, early 2000s), as well as the current parent of a kid in a very different (Midwestern) city's public school systems, this is the answer I would point to. Teacher quality has always been a distribution, with some good and bad outliers, but the vast majority are sufficient. What hasn't always been the case is the inability to get rid of kids who are extremely disruptive and make it impossible for education to take place. There also seems to be a bigger share of that disruption now than two decades ago, when I was teaching. I think a lot of it relates to feedback loops: in the suburban schools in my area, as well as the elite public high school in my area (requires an academic test for entrance), these problems don't exist. The problems only seem to exist in the schools where there are higher concentrations of antisocial expectations/behavior. This does tend to correlate with poverty, but poverty doesn't apprar to be causative, merely correlated: I would point out that poor kids who test into the elite public high school tend to do just fine. But, they are all there for a reason (i.e., it is a self-selecting group of people who want to test), and also worked hard enough to pass the entrance exam.