Does the impact of teachers extend beyond the students in their classroom? Using the natural transitions of students from multiple elementary schools into a single middle school, this paper provides a new method for isolating and quantifying peer spillover effects of teaching and shows that ignoring these spillovers underestimates a teacher’s value by at least 30 percent. Because the spillovers also affect teacher value-added estimates, I develop a method of moments estimator of teacher value-added and show that accounting for the spillovers does not have a large impact on the ranking of teachers in New York City. I conclude by showing that the spillovers occur within groups of students who share the same race and gender, which suggests that social networks play a critical role in disseminating the effect. (JEL H75, I21, J15, J16, J45, Z13)

We model optimal treatment assignment in programs with a limited number of seats and study how the presence of peer effects impacts the optimal allocation rule. We then use data from a randomized control trial to show evidence that there are large peer effects in the context of job training for disadvantaged adults in the United States. Finally, we combine the model and the empirics to show that the program would have had a much greater impact if the assignment choices had accounted for the peer effects.