Here is the abstract [HT: Greg Mankiw]:
Most analyses of teacher quality end without any assessment of the economic value of altered teacher quality. This paper combines information about teacher effectiveness with the economic impact of higher achievement. It begins with an overview of what is known about the relationship between teacher quality and student achievement. This provides the basis for consideration of the derived demand for teachers that comes from their impact on economic outcomes. Alternative valuation methods are based on the impact of increased achievement on individual earnings and on the impact of low teacher effectiveness on economic growth through aggregate achievement. 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.
There is a big problem however: how do you measure a good teacher, or even more problematically, how do you measure good performance on an annual basis? In Brazil I heard about experiments in Chile with so-called value added measures of teacher quality that were terrible disappointments. There is a complete lack of consistency across different tests and even within sections of the same test. In other word, when they gave a standardized math test to students at the beginning and at the end of a year, some students improved markedly on one section of the test but did not on the other and vice-versa. So there was no clear way to rank classrooms in terms of the value added by the teacher. The same was true across tests (Spanish and Math, say). So how can you say one teacher is better than others? You can't.
We are still a long way off from the answers.