Exactly right, UCGal
Teachers should be evaluated on how much progress they make with their students each calendar year, not simply the average of their class at the end of the year. Call it value added, and then pay accordingly.
Teachers tend to want the easy job of working with well-to-do suburban kids with all the advantages that implies. It is easier and more fun. Naturally the higher seniority teachers gravitate to those schools, and the union rules enable this practice.
In the meantime, the truely disadvantaged inner city and minority schools are hurt by the status quo. It is interesting how unions, and the liberals that support their cause end up hurting the poor and helping the well-off.
I suggest that a teacher that inspires and helps a class of losers in a weak school go from awful to merely average in an academic year should be paid accordingly. Say, twice what an average teacher makes today. And an underperforming teacher in a wealthy school should have their next year’s pay reduced. This might encourage them to leave teaching, an added bonus. Of course, unions would oppose such an incentive-based system.