It is true that you don’t want to rank the teacher based on the crumbling foundation built by all those that came before. BUT, when you entrance then exit test the teacher’s students and calculate the delta, you have a better measure of teacher performance. Plus you can discount the likely limited achievement of those with poor entrance scores to mitigate the legacy penalty.
But this doesn't address the fundamental cultural problem present in many bad schools. In most inner city schools, most students simply do not want to learn and tend to have more disciplinary problems that get in the way of any actual instruction. You could have the world's best teacher trying to work in such an environment and barring some miracle, or a change in cultural mindset, there won't be an overall change in test scores.
IMO, the only solution to dealing with bad schools is to make it easier for the students who do want to learn to leave them. Or, make it easier to kick out the disruptive students who do not want to learn so that the remaining students can learn in peace.