This paper studies teacher attrition in Wisconsin following Act 10, a policy change which severely weakened teachers' unions and capped wage growth for teachers. I document a sharp increase in turnover after the Act was passed, driven almost entirely by the exit of older teachers, who faced strong incentives to retire before the end of pre-existing collective bargaining agreements. I find that student academic performance increased in grades with teachers who retired following the reform, and I obtain similar results when instrumenting for retirement using the pre-existing age distribution of teachers. Differences in value-added between retirees and their replacements can potentially explain some, but not all, of the observed academic improvements.