Handwashing is key to preventing the transmission of various infectious diseases of which school-aged children are particularly susceptible. Traditional, information-messaging campaigns may increase handwashing awareness but have had limited success in promoting behavior change. Behavioral economics "nudges," which explicitly target the knowledge-behavior gap, is a promising alternative. We evaluate the impact of school-based nudges in the first fully powered cluster randomized controlled trial in the Philippines. Out of our sample of 132 eligible schools, we randomly assigned half to receive nudges, including contextual cues (painted footpath from toilet to handwashing station) and visible reminders (posters and eye sticker), and half to the control group. Four months after implementation, we measured handwashing with soap (HWWS) after toilet use among grades 1-6 students using direct observation and compared this outcome between treatment and control schools. We also assessed whether nudges increased soap availability. The intervention increased HWWS rates by 17.3% points (pp), [95% CI: 4.2, 30.4] in treatment schools from the control group mean of 11.7%. The effect size was comparable across gender and age groups. Access to functioning handwashing facilities with soap increased by 36% (+20.2 pp, 95% CI: 10.9, 29.4). Mediation analysis suggests the program simultaneously nudged students to wash hands with soap in classrooms that already had soap, and nudged teachers to provide soap where it was not already available. These findings demonstrate that behavioral nudges costing less than $70 per school can lead to significant increases in HWWS among students 4 months post-intervention.