How much does a bad teacher cost a student?

A judge’s decision in Los Angeles today will spark a renewed debate over teacher tenure.

Judge Rolf M. Treu struck down California’s teacher tenure law, agreeing with plaintiffs who said it makes it impossible to get rid of low-performing and incompetent teachers.

The decision acknowledged that the worst teachers get assigned to the worst schools, depriving students in poverty the right to an equal education.

In his decision, Treu relied heavily on the testimony of Harvard economist Raj Chetty, who claimed in his testimony that a bad teacher costs a student $1.4 million in lifetime earnings.

California is one of the few states adopting a two-year period before tenure is bestowed. Treu said three to five years is a better period to fairly evaluate a teacher’s ability. Minnesota, incidentally, has a three-year probationary period for teachers.

Judge Treu said there are currently 350 “grossly ineffective” teachers in Los Angeles alone.

For fans of This American Life, it’s difficult to read the decision without thinking of the Rubber Room, profiled by Ira Glass in a 2008 show about teachers who were under investigation and couldn’t be fired by New York City.

The rubber rooms were abolished in 2010.