If you're interested in knowing what is a predictor of being a good teacher, you should talk to my mom. She is a principal at a small school in rural Texas, where she has a high number of students with low socioeconomic status and from severely broken homes (drugs, absentee parents, abuse, etc.) She came into the system with a number of older teachers whom, having worked with them as a teacher for several years before becoming the principal, she knew were weak teachers. Because it's a small town, every year or two someone comes through who was a former Miss Small Town in our town or who is a niece, nephew, or close family friend of someone on the school board, and she is expected to hire them. While she's put her foot down a few times, she knows that it's best not to rock the boat too much, not least of all because it could mean her job.
Despite this, she has excellent test scores and frequently comments on the quality of her teachers. She attributes this not only to hiring, but also to her skill in recognizing her teachers' strengths and weaknesses. Before she became principal, the same teachers had been teaching the same classrooms for nearly 20 years. After a couple of years, though, she began to suggest moving teachers around, putting strong teachers in classrooms where they were most needed (where tests were most rigorous and with special ed students) and putting her weak teachers either in classrooms where they would be happier, and thus work better, or where their impact would be less severe. While it doesn't necessarily solve the problem of poor teachers, it helps to minimize the impact on students, and I think it also helped by making her teachers happier and more effective in what they were doing.
Having good teachers means having a good support system, having someone shrewd enough to understand how each teacher works and how each teacher could be utilized best. You have to get to know the people once you've hired them. A good principal does more than just kick out the ones who aren't performing up to snuff -- he tries to figure out how he can make them stronger teachers. Often, unless the person is just plain lazy or completely apathetic (something few new teachers are) that person will rise up to meet the requests for a better performance or trying something new or using new methods in the classroom. Some teachers have a gift for teaching and are naturally good at what they do. Most aren't, though, and need guidance and a little bit of molding. This is what principals are supposed to be there for.
Something else my mom does when hiring new teachers -- she looks at their grades in college. So many people say that this is not a strong determinant of how people will do professionally, but my mom's theory has always been either they're incredibly bright or they're incredibly hard-working, and either way, those are raw materials she can work with. She's never been wrong on that score.
