Collective leadership of teachers and administrators can be a vehicle for catalyzing school improvement. In chemistry, a catalyst is any substance that increases the rate of reaction without itself being consumed. Leadership that accelerates good work without using up the leader is increasingly necessary. We identified schools that demonstrated exemplary Science, Technology, Engineering, and Mathematical (STEM) learning. Using a theoretical model of collective leadership development, we conducted a multiple-case study to identify common themes that provided insight into school improvement. We surveyed STEM leaders (n=113), conducted interviews and focus groups with teachers (n=52) and administrators (n=18), and conducted site visits to five schools. We found seven implications for policymakers: (1) enact policies that support site-based leadership; (2) implement professional learning of teachers and administrators together on work related to shared goals; (3) support peer observation and feedback; (4) provide opportunities for administrators, teachers, and students to design flexible schedules that support cross-curricular STEM connections; (5) develop public/private partnerships that can provide expertise and materials; (6) create opportunities for educators and students to make their work public; (7) engage school-based teams in iterative improvement cycles that rely on collection of observable evidence of improvement using engineering and design principles.