Current policy efforts that seek to improve learning in science, technology, engineering, and mathematics (STEM) emphasize the importance of helping all students acquire concepts and tools from computer science that help them analyze and develop solutions to everyday problems. These goals have been generally described in the literature under the term "computational thinking." In this article, we report on the design, implementation, and outcomes of an after-school program on computational thinking. The program was founded through a partnership between university faculty, undergraduates, teachers, and students. Specifically, we examine how equitable pedagogical practices can be applied in the design of computing programs and the ways in which participation in such programs influence middle school students' learning of computer science concepts, computational practices, and attitudes toward computing. Participants included 52 middle school students who voluntarily attended the 9-week after-school program, as well as four undergraduates and one teacher who designed and implemented the program. Data were collected from after-school program observations, undergraduate reflections, computer science content assessments, programming products, and attitude surveys. The results indicate that the program positively influenced student learning of computer science concepts and attitudes toward computing. Findings have implications for the design of effective learning experiences that broaden participation in computing.