Abstract Deviant peer affiliation is one of the most important predictors of alcohol use in adolescence. These affiliations often arise when socially marginalized youth self-aggregate and reinforce alcohol use and other deviant activity (i.e., ?deviant peer clustering?). Even though the field has developed efficacious school-based prevention programs, program effects are generally small, and these programs can be difficult to disseminate with fidelity and challenging to sustain due to complex designs and significant time-and-money expenditures required for materials and training. Thus, existing school-based prevention programs have not provided compelling value to schools, which has limited their dissemination. In a recent RCT, we aimed to reduce alcohol use in middle school by exposing at-risk youth to a broader cross-section of the school social network (and interrupting the process of deviant peer clustering) through collaborative, group-based peer learning activities. We found significantly lower rates of deviant peer affiliation and alcohol/tobacco use in intervention schools as compared to control schools. Peer learning also had moderate-to-strong suppressive effects on bullying, victimization, stress, and emotional problems, as well as strong positive effects on student engagement and achievement, suggesting that it can promote positive behavioral, social, and academic outcomes simultaneously. However, teachers in our intervention schools encountered challenges when implementing peer learning, including: (1) design fidelity, i.e., ensuring that peer learning provided the most positive student experience by including all the essential design elements; and, (2) instructional support, i.e., managing the flow and timing of the activities to complete the lesson on time while dealing with unexpected disruptions. In our Phase I SBIR, we developed an initial version of a mobile software application (PeerLearning.net) that provided easy-to-use organizational templates with workflow support that teachers used to automate the design and delivery of peer learning lessons. During the pilot of the app, we found that it exceeded benchmarks for usability, satisfaction, and sustainability; teachers felt confident when using the app, and students reported a high degree of engagement and enjoyment. In this Phase II project, we will expand the number of lesson templates available in the app and add dashboard reporting capabilities for teachers and administrators. With these new capabilities, the app should be highly attractive to teachers (for its ease of use, rapid implementation, flexibility, and instructional support) and school administrators (for its evidence-based approach to the prevention of a variety of behavioral problems and greater student motivation and achievement). By meeting a variety of different needs at different levels in the educational system, PeerLearning.net will present a compelling value proposition to schools and districts. Once the value of PeerLearning.net is recognized, we anticipate a high rate of adoption, enabling us to spread evidence-based alcohol use prevention concepts in a flexible, scalable, easy-to-use framework. This widespread dissemination should create a rapid, sustainable impact on public health among adolescents.