Despite the digital saturation of today's youth across demographic groups, students of color and females remain severely underrepresented in computer science. Reporting on a sequential mixed methods study, this article explores the ways that high school computer science teachers can act as change agents to broaden the participation in computing for historically underrepresented students. Three high school case studies reveal a critical need for professional development and support to do this work. The subsequent part of the study focuses on the impact of a district-university intervention which trained 25 urban teachers to teach Advanced Placement computer science in their schools. The swift success of this intervention was evident from the following years' dramatic increase in course offerings and enrollment of females, Latinos, and African Americans. (Contains 2 figures and 2 tables.)