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.
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