Catherine Rampell of the New York Times flags a new study by the Hoover Institution’s Eric Hanushek, in which he tries to assess the economic impact of effective teachers: 

A teacher one standard deviation above the mean effectiveness annually generates marginal gains of over $400,000 in present value of student future earnings with a class size of 20 and proportionately higher with larger class sizes. Alternatively, replacing the bottom 5-8 percent of teachers with average teachers could move the U.S. near the top of international math and science rankings with a present value of $100 trillion.

The numbers cited both overstate the value of quality teachers and understate the magnitude of the problems caused by bad teachers. 

$400,000 in present value of student future earnings with a class size of 20 means that each student is earning an extra $20,000 over his lifetime. Assuming that each student works for 40 years, that’s only a present-value difference of $500 per year per student.

The $100 trillion aggregated number probably makes more sense, intuitively. The value of a good teacher isn’t so much in a adding slightly to the knowledge and capabilities of one students, for example raising the present value of their lifetime earnings by $20,000. Instead, the real benefit teachers can provide is in changing students’ outcomes from failure to success. For instance, a teacher who causes a student to graduate from college instead of dropping out of high school has a massive effect on that student’s economic future. The bottom 5-8 percent of teachers are the ones who can’t or don’t have the ability to do that. 

Yet replacing those 5-8 percent of teachers is an impossibly tall order. Michelle Rhee fired 6 percent of D.C. Public School. Now she is a national hero, precisely because she is the only one to have accomplished such a feat. Also, her heroics cost her her job and possibly her boss’s. Unfortunately, there is no guarantee that those teachers will be replaced with average teachers. We don’t know for sure, but average might turn out to be pretty good. 

To replicate Rhee’s hard-won accomplishments on the national level in a politically sustainable way would involve firing hundreds of thousands of teachers, and politically defeating the teachers’ unions and, by extension, the Democratic Party. In other words, thinking about replacing the bottom 5-8 percent of teachers with average teachers is, for now, simply a thought experiment, not a possible agenda item.