The TechPALS project expanded a general-purpose handheld CSCL tool (from Chile) to a 3-week classroom module for primary school mathematics (in the United States). To go from tool to module we articulated a framework for an effective CSCL practice– including curricular fit, training materials, pedagogical guidance, formative and summative assessments, and logistical support. In parallel, to meet requirements of the U.S. Department of Education, we conducted classroom experiments to investigate the achievement differences between students who were randomized to use either TechPALS or a non-CSCL product. In this paper, we examine the design changes from initial classroom pilot tests to eventual attainment of statistically significant results, emphasizing the integration of technology, activity designs, and broader educational practices that was required to achieve impacts in ordinary, low-income schools. Based on these results, we recommend a “curricular activity system” framework to support effective CSCL practices.