We knew it all along, that teachers with more seniority get to choose where they teach, and they choose to teach better students, leaving the newer teachers to the poorer schools.
Now it's been quantified (PDF). The Center for the Future of Teaching and Learning talked to principals about the number of teachers who don't meet paper qualifications for their job. There are fewer than in 2000 (11,000, down from 42,000), but they are concentrated in lower-performing schools. As I said, no surprise, but it's nice to have it pointed out again, as in this LA Times article.