Comprehensive sexuality education may help prevent intimate partner violence, but few evaluations of sexuality education courses have measured this. Here we explore how such a course that encourages critical reflection about gendered social norms might help prevent partner violence among young people in Mexico. We conducted a longitudinal quasi-experimental study at a state-run technical secondary school in Mexico City, with data collection including in-depth interviews and focus groups with students, teachers, and health educators. We found that the course supported both prevention of and response to partner violence among young people. The data suggest the course promoted critical reflection that appeared to lead to changes in beliefs, intentions, and behaviors related to gender, sexuality, and violence. We identify four elements of the course that seem crucial to preventing partner violence. First, encouraging participants’ reflection about romantic relationships, which helped them question whether jealousy and possessive behaviors are signs of love; second, helping them develop skills to communicate about sexuality, inequitable relationships, and reproductive health; third, encouraging care-seeking behavior; and fourth, addressing norms around gender and sexuality, for example demystifying and decreasing discrimination towards sexually diverse populations. The findings reinforce the importance of schools for violence prevention and have implications for educational policy regarding sexuality education. The results suggest that this promising and relatively short-term intervention should be considered as a school-based strategy to prevent and respond to partner violence.