Social contagion has been documented across various domains. However, this phenomenon has not been explored in relation to formative assessment in schools. This study examines the social contagion of school teachers' formative assessment practices and self-efficacy. A sample of 296 teachers from 12 Hong Kong primary and secondary schools participated in this study. All participants completed the same questionnaire twice, one year apart. The longitudinal results showed that formative assessment practices and self-efficacy were socially contagious. When teachers had colleagues who engaged in formative assessment practices and had high self-efficacy, they were more likely to engage in formative assessment and become more efficacious themselves. Also, school-level formative assessment self-efficacy predicted individual-level formative assessment practices demonstrating the importance of collective efficacy. The findings can inform the provision of school support and the design of teacher professional development programmes for promoting formative assessment in schools.