I thought this Slate.com article was very interesting.

Firing bad teachers may seem like a rather obvious solution, but it requires some gumption to take on a teachers union. And cleaning house isn't necessarily the only answer. There are three basic ways to improve a school's faculty: take greater care in selecting good teachers upfront, throw out the bad ones who are already teaching, and provide training to make current teachers better. In theory, the first two should have more or less the same effect, and it might seem preferable to focus on never hiring unpromising instructors—once entrenched, it's nearly impossible in most places to remove teachers from their union-protected jobs. But that's assuming we're good at predicting who will teach well in the first place.

It turns out we aren't. For instance, in 1997, Los Angeles tripled its hiring of elementary-school teachers following a state-mandated reduction in class size. If L.A. schools had been doing a good job of picking the best teachers among their applicants, then the average quality of new recruits should have gone down when they expanded their ranks—they were hiring from the same pool of applicants, but accepting candidates who would have been rejected in prior years. But as researchers Thomas Kane and Douglas Staiger found, the crop of new teachers didn't perform any worse than the teachers the school had hired in more selective years.

This unexpected result is consistent with the findings from dozens of studies analyzing the predictors of teacher quality. Researches have looked at just about every possible determinant of teaching success, and it seems there's nothing on a prospective teacher's résumé that indicates how he or she will do in the classroom. While some qualifications boost performance a little bit—National Board certification seems to help, though a master's degree in education does not—they just don't improve it very much.

I'm not sure this result is really that surprising. To me, good teaching is a bit science and a bit art. You can't tell is someone is a talented singer or violinist based on their resume. You have to hear them play. I believe that teaching is the same. It is is difficult to determine if someone is a good teacher solely based on their resume, an interview and a couple reference calls.

Since firing a bad teacher is almost impossible, that's one reason why I support lengthening the time that it takes for teachers to gain tenure. With just two years of time, there really isn't much time for intervention to help poor teachers improve. In the present system, a principal only has a couple years to assess the teacher's skills and help them to improve their classroom skills before they have to make a judgement on whether they should continue teaching. I think beginning teachers need and deserve the time to perfect those skills.