This paper develops a conceptual framework for how cooperative learning in multicultural Social Studies (IPS) classrooms might foster Putnamian social capital intergroup trust, civic reciprocity, and democratic participation among adolescent students in heterogeneous schools. Rather than reporting findings from an original survey or focus-group study, the analysis synthesizes Robert Putnam's (1993, 2000) social capital framework with the cooperative-learning literature (Johnson & Johnson, 2009) to derive a set of theoretically grounded propositions linking specific dimensions of cooperative learning quality to specific dimensions of social capital formation, and to the moderating role of classroom heterogeneity. The framework proposes that cooperative learning quality is associated with social capital formation, with intergroup trust expected, on contact-theory grounds, to be the most foundational dimension, followed by civic reciprocity and democratic participation. Classroom heterogeneity is proposed to moderate the cooperative-learning-to-trust pathway, such that the trust-building function of cooperative structures is amplified when diversity is structurally embedded in task design. Three candidate mechanisms role rotation, accountable interdependence, and deliberative dialogue are proposed as the processes through which cooperative structures might translate into Putnamian social capital. The paper offers this model as a heuristic for future empirical research, together with a set of testable propositions rather than confirmed findings, with implications for IPS curriculum development and teacher training in pluralistic educational contexts.