Bullying is a significant public health problem for students in schools. Prevention programs have addressed targets with some success; however, meta-analyses find small effects among older youths. A pilot study was conducted with high school students to evaluate the potential efficacy of StandUp, a three-session online program that delivers assessments and individualized guidance matched to bullying experiences and stage of readiness for using healthy relationship skills. Of the 113 students participating in the study, 88 completed all three intervention sessions. Use of healthy relationship skills increased significantly from session 1 to session 3. In addition, compared with session 1, participants at session 3 had reduced odds of perpetrating and experiencing emotional and physical bullying, and of passively standing by as others were bullied; odds ratios ranged from 0.29 to 0.63, with most bullying outcomes approaching or reaching statistical significance. StandUp is a bullying prevention program for adolescents that showed encouraging changes in behavior from first to third session in a small sample of high school students. Because it is easy to use, guarantees fidelity of administration, and does not require extensive staff time, StandUp may be a useful addition to the array of school-based programs to address bullying.