Handwashing with soap is an important preventive health behavior, and yet promoting this behavior has proven challenging. We report the results of a program that trained teachers to deliver a handwashing with soap behavior change program to children in primary schools in Bihar, India. Ten intervention schools selected along with ten nearby control schools, and intervention schools received the “School of Five” program promoting handwashing with soap using interactive stories, games, and songs, behavioral diaries to encourage habit formation, and public commitment. Households with children aged 8–13 attending the nearby school were enrolled in the study. Handwashing with soap was measured using sticker diaries before eating and after defecation 4 weeks after the intervention was completed. Children in the treatment reported 15.1% more handwashing with soap on key occasions (35.2%) than those in the control group (20.1%) (RR: 1.77, CI: (1.22, 2.58), p = .003). There was no evidence that handwashing with soap after defecation was higher in the treatment group than the control group (RR: 1.18, CI: (0.88, 1.57), p = .265), but there was strong evidence that handwashing with soap was greater in the treatment than in the control before eating (RR: 2.68, 95% CI: (1.43, 5.03), p = .002). Rates of handwashing increased both at home (RR: 1.63, CI: 1.14, 2.32), p = .007) and at school (RR: 4.76, 95% CI: (1.65, 17.9), p = .004), though the impact on handwashing with soap at key occasions in schools was much higher than at home. Promoting handwashing with soap through teachers in schools may be an effective way to achieve behavior change at scale.