Unfortunately, it's not easy to determine what makes a good teacher, except in retrospect.
“There’s been a lot of research on what exactly makes a good teacher,” Friedman says. “Frankly, it hasn’t been all that helpful. There’s one predictor of value-added, which is teacher experience. In the first couple of years, teachers’ value-added goes up quite a bit. Beside that, people who have more-advanced degrees, [have] higher SAT scores, graduated from a better college, are certified versus uncertified — none of these things are strong predictors of value-added.”