Effective teachers are best identified by their performance, not by their background or experience.
Despite common perceptions, effective teachers cannot reliably be identified based on where they went to school, whether they're licensed, or (after the first few years) how long they've taught. The best way to assess teachers' effectiveness is to look at their on-the-job performance, including what they do in the classroom and how much progress their students make on achievement tests. This has led to more policies that require evaluating teachers' on-the-job performance, based in part on evidence about their students' learning.
Effective teachers tend to stay effective even when they change schools.
Recent evidence suggests that a teacher's impact on student achievement remains reasonably consistent even if the teacher changes schools and regardless of whether the new school is more or less advantaged than the old one.