There’s now solid evidence that there are huge differences in the effectiveness of teachers, even within high-poverty schools. The gold standard study, by Harvard and Columbia University scholars and released in December by the National Bureau of Economic Research, took data from a major urban school district and found that even in the context of poverty, teachers consistently had a huge positive or negative impact.

Get a bottom 1 percent teacher, and the effect is the same as if a child misses 40 percent of the school year. Get a teacher from the top 20 percent, and it’s as if a child has gone to school for an extra month or two.

The study found that strong teachers in the fourth through eighth grades raised the game of their students in ways that would last for decades. Just having a strong teacher for one elementary year left pupils a bit less likely to become mothers as teenagers, a bit more likely to go to college and earning more money at age 28.

Removing the bottom 5 percent of teachers would have a huge impact. Students in a single classroom with an average teacher, rather than one from the bottom 5 percent, collectively will earn an additional $1.4 million over their careers, the study found.

Another study, one from Los Angeles that has been contested, suggested that four years in a row of having a teacher from the top quarter of teachers, instead of from the bottom quarter, might be enough to erase the black-white testing gap.

How does one figure out who is a weak teacher? Yes, that’s a challenge. But researchers are improving systems to measure “value added” from beginning to end of the year, and, with three years of data, it’s usually possible to tell which teachers are failing.