PowerTeaching is an evidence-based, structured cooperative learning program. It aims to prepare students to meet the stringent demands of new state math standards, both the knowledge standards and the 21st-century skills standards, such as around communication and collaboration. Awarded a scale-up grant in 2012 by the federal government, the program expanded to more than 100 high-need middle schools to train their math teachers how to implement cooperative learning in their classrooms. The evaluation of this expansion effort found that while program teachers learned to organize their students into long-standing mixed-ability groups, which are conducive of cooperative learning, teachers did not consistently implement the instructional supports necessary to transform group work into cooperative learning. The experimental impact study found that students' math performance did not differ dramatically between program and non-program schools. A likely cause for the weak implementation was that the ongoing professional development that is an integral part of PowerTeaching mostly did not occur or was more focused on how to teach the new content required by recently adopted education standards rather than how to implement cooperative learning. The impacts were not large and significant (as earlier studies had found), in part because students in the control schools were also working in groups in response to the communication and collaboration standards embedded in the new standards. Indeed, this study is one of the first to document this shift toward group work. Given that prior studies have shown that cooperative learning teams increase the academic performance of students more than group work does, the evaluation points to the potential for increasing math performance if teachers, who are now routinely using groups, can be trained to consistently provide the supports needed to transform groups into cooperative learning teams. This type of ongoing support, missing in this trial of PowerTeaching, is likely to be key to shifting teachers' instructional practices. [This report was written with Deni Chen, Ashley Kennedy, and Joseph Quinn.]