Plenty of teachers are willing to work in an environment where they feel they could make the biggest impact. My mother spent her whole career working in schools that could be classified as inner city with high percentages of new immigrants. Not everyone wants the easiest job possible in their field, some people see rewards in being able to make a bigger difference in people's lives.
My point is don’t actively screw those people by coming up with a dumb compensation formula that punishes them for students underperforming before they ever entered their class. If you have merit based pay you need to evaluate how much a student improved under a specific teacher’s watch.
It wouldn’t if the obvious consequence of the bluntness was to chase good teachers out of the schools that most need them. And that’s what a blunt policy that punishes teachers for having students enter their class at a low level does.