This paper describes the social architecture model of school-based bullying behavior. The model proposes that the behavior of all students affects rates of bullying. Alongside self-reported victims and bullies, the model identified four bystander roles: assistant, reinforcer, outsider, and defender. The level of support for bullies varies based on school policies that address bullying and promote school connectedness. The universal components of the KiVa school-based anti-bullying program designed to teach pupils to stand against bullying are described. The Stand Together trial, a UK-based randomized controlled trial, recruited 11,000+ students from 118 schools across the UK, half of whom received the KiVa program whilst the remainder delivered usual practice to address bullying. The main trial results reported a significant reduction in victimization in favor of KiVa. This paper examines data collected on the pupil-reported Participant Role Questionnaire (PRQ), one of the secondary measures used to explore whether significant reductions in victimization were accompanied by changes in bystander behavior. The results showed reductions in the student response rates of self-identified roles as bullies, assistants, and reinforcers in favor of KiVa, but outsider roles increased, and defender roles reduced. This provides tentative support for the social architecture model as taught in the Stand Together KiVa trial but also suggests that further work needs to be conducted to support the development of defender behaviors and address this important public health challenge.