North American teacher unions' positive contributions to educational change have historically flown under the radar of educational policy makers, a situation that has been reified by recent attacks on public sector unions. In this article, I draw on social movement theory and an institutional case study of a self described social justice union to analyze teacher unions' unique contributions to educational improvement. The primary product of this analysis is a cyclical model of union-supported, teacher-initiated educational change that codifies 40 years of social justice activism incubated, supported and catalyzed by a Canadian teacher union.