Anecdotally, the schools with the worst funding also have the most troubled students. I think many new teachers are up for a challenge and low pay at first, but are disturbed by the reality of what they find and seek better schools for their own mental wellbeing. From what I've heard, these schools get a steady supply of freshly minted young teachers straight out of college, who burn out after a few years. Their teacher supply problems come from a failure to retain teachers, not an inability to hire them in the first place.