Using a unique data set on teachers’ union election certifications from Iowa, Indiana, and Minnesota, I estimate the effect of teachers’ unions on school district resources and on student educational attainment. My empirical strategy allows for nonparametric leads and lags of union age. I find no impact on teacher pay or per student district expenditures but that unions increase teacher employment by 5%. I find no class size effect because of enrollment increases in unionized districts, and I estimate that unions have no net effect on high school dropout rates. These findings highlight the importance of correctly measuring unionization status.