Its important to point out that school teaching is a churn and burn field. 17 to 19 years of training followed by a half life of 6 years in the classroom. Not a recent problem either, goes back decades. If your career is shorter than a pro football player, no need for continuous development on a large level, or "most teachers sit in a classroom for a day" is enough if most of them will be out of the field in a couple years regardless.

The career structure, much like the rest of the country, is extremely pyramidical. So teaching in the slums requires a clean(ish) criminal record and frankly not much else, at least to get hired. On the other hand, civilized low crime areas are hyper competitive. So training and advanced degrees translate into nicer working environment.

There will be survivors even 3 or 4 half lives out in teaching, much like programming, but use em up and throw em away as a national policy does not encourage serious system wide training efforts.