There's an interesting article in the Atlantic touting some K-12 teaching effectiveness data of Teach for America.

Superstar teachers had four other tendencies in common: they avidly recruited students and their families into the process; they maintained focus, ensuring that everything they did contributed to student learning; they planned exhaustively and purposefully—for the next day or the year ahead—by working backward from the desired outcome; and they worked relentlessly, refusing to surrender to the combined menaces of poverty, bureaucracy, and budgetary shortfalls.

The observation about relentlessness is a bit vague. Is it any different from observing that harder workers tend to be better workers? The next quote seems more clear-cut.

Knowledge matters, but not in every case. In studies of high-school math teachers, majoring in the subject seems to predict better results in the classroom. And more generally, people who attended a selective college are more likely to excel as teachers (although graduating from an Ivy League school does not unto itself predict significant gains in a Teach for America classroom). Meanwhile, a master’s degree in education seems to have no impact on classroom effectiveness.

To be fair to the Ed schools, the people that apply to Teach for America are not a random sample of all teachers, so it's conceivable that the non-correlation above is spurious. Is there a randomized study measuring the effect of a master's in education?