Schoolwide interventions are among the most effective approaches for improving students' behavioral and academic outcomes. However, researchers have documented consistent challenges with implementation fidelity and have argued that school social workers should be engaged in efforts to improve treatment integrity. This study examines contextual influences on the implementation of a whole-school intervention called Responsive Classroom (RC) in one urban K-8 public school serving a diverse student body. RC improves social, emotional, literacy, and math outcomes for disadvantaged students with behavior problems by building on the assets of teachers to intervene with misbehaving students in the classroom setting or school environment. Yet little is understood regarding the factors that constrain or enable implementation of RC in noncontrolled research conditions. Results from a mixed-methods convergent analysis of focus group, observation, and survey data indicate the influence of the following three contextual factors on implementation fidelity: (1) intervention characteristics such as compatibility with staff members' beliefs about behavior change and management, (2) organizational capacity such as principal and teacher buy-in, and (3) the intervention support system such as training and technical assistance. Implications for future school social work research and practice with respect to the implementation of schoolwide programs are discussed.