This study investigates the effectiveness of the Responsive Classroom (RC) approach, a social and emotional learning intervention, on changing the relations between mathematics teacher and classroom inputs (mathematical knowledge for teaching [MKT] and standards-based mathematics teaching practices) and student mathematics achievement. Work was conducted in the context of a randomized controlled trial. Participants were 88 third-grade teachers and their 1,533 students. A multigroup path analysis accounting for fidelity of implementation revealed no direct or indirect effects linking MKT to student achievement in the RC or control condition. The same analysis revealed different findings for the RC versus control teachers. In the RC group only: (a) Teachers trained in RC who used more RC practices showed higher use of standards-based mathematics teaching practices, and (b) higher use of standards-based mathematics teaching practices related to greater improvements in math achievement. No comparable findings were evident in the control condition. Results demonstrate the importance of building social and emotional capacity in teachers by helping create a supportive classroom that helps teachers provide stronger mathematics teaching practices that lead to improved student learning.