Students' academic engagement is greatly informed by a classroom's social climate. However, more research is needed regarding how specific peer behavior, especially prosocial behavior, come to shape academic engagement. The present study investigated whether students' perceptions about their classmates' prosocial behavior were associated with their academic engagement (cognitive, behavioral, affective) across the school year. Indirect effects via increases in students' own prosocial behavior were examined. Participants were 905 middle school students from rural, low-income communities in the Midwestern United States (50% girls, 46% boys; M[subscript age] = 12.94 years). Students completed self-report surveys in the fall and spring of the 2022-2023 school year. Results revealed that students' perceptions of their classmates' prosocial behavior were positively associated with students' own prosocial behavior. Students' own prosocial behavior was positively associated with all three dimensions of engagement. The positive indirect effect of classmates' prosocial behavior on engagement through students' own prosocial behavior was significant. The findings highlight the importance of classmates' behavior on individuals' academic engagement and offer insights into classroom-based interventions aimed at improving collective behavior. [This is the online first version of an article published in "Journal of Youth and Adolescence."]