["Psychology in the Schools, Volume 63, Issue 6, Page 1001-1019, June 2026. ", "\nABSTRACT\nThis study reports on the development, factor structure, and validity indicators of the Prosocial Behavior in School Scale (PBSS) among 4264 sixth‐ to eighth‐grade students in the United States. The final PBSS is a 12‐item self‐report measure of students' prosocial behavior in a specific classroom. The PBSS was developed using student descriptions of authentic prosocial behaviors. Factor analysis suggested that responses to the PBSS were represented by two distinct but correlated factors–Responsive Prosocial Behavior (PB) and Principled PB. These factors were invariant across gender, grade levels (sixth to eighth), and ethno‐racial groups, and demonstrated internal consistency. Small gender, ethno‐racial, and grade‐level differences emerged in frequency of self‐reported prosocial behavior. Validity of the PBSS was supported by measures of antecedents and outcomes of prosocial behavior. Specifically, PBSS scores correlated positively with social connection (peer friendships and teacher–student relationships), in‐class engagement over the school year, and teacher use of praise (but not use of induction). Implications for research and practice are discussed."]