Safe School Ambassadors (SSA) is a student-centered bystander education program developed by Community Matters to reduce bullying/mistreatment and enhance school climate. A two-part program evaluation was designed to assess the efficacy of the program's logic model and its impact on school-level discipline indicators. For the first part, a two-year evaluation was conducted in five middle schools in Texas using a quasi-experimental pre-post-post design. After two years, rates of helpful intervention were higher for male Ambassadors than for their controls. Ambassadors' Friends noticed more helpful interventions and reported observing less mistreatment than Friends of Key Students at the control schools. Process data indicated positive effects on discipline and overall climate. For the second part, discipline data were gathered from 19 schools that implemented the SSA program with a high degree of fidelity to internal benchmarks and from demographically matched non-SSA schools that served as controls. Analysis showed statistically significant reductions (p less than 0.05) in suspensions and other discipline indicators at SSA schools over time and as compared to control schools. (Contains 3 tables and 3 figures.)