Bullying bystanders' reactions are important for either stopping or perpetuating bullying behaviors. Given school-based bullying programs' focus on bystanders, understanding the associations between school-level factors and individual bystander responses can improve intervention efficacy. Data from 64,670 adolescents were used to examine bullying bystander responses as a function of 13 school-climate dimensions within 3 main factors (Engagement, Environment, Safety) and individual-level factors (e.g., race/ethnicity, perceptions of student-teacher connectedness). Multi-level models showed schools with better Engagement and Safety had higher odds of defender behaviors, a better Environment was associated with lower odds of passive and assisting behaviors. Differences also varied by individual-level factors. For example, an aggressive climate was associated with passive behaviors more strongly in boys and high schoolers. Further, higher perceived parent-teacher and student-teacher connectedness were associated with positive bystander behaviors, and this was stronger for Black and Latinx youth, highlighting the importance of improving relationships as a crucial starting point. [This paper was published in "Journal of Applied Developmental Psychology" v80 Article 101412 2022.]