This paper describes the implementation and student learning outcomes of a nationwide professional development program for lower secondary and upper secondary school teachers to integrate computational thinking into the K-12 curriculum. Computational thinking comprises important concepts and skills that all students should develop to take an active role in a global society. However, teaching computational thinking is challenging. There are few teachers with the knowledge and skills to integrate computation into their courses. In this program, the participating teachers implemented a set of lesson plans that included both unplugged activities to scaffold student learning, and 'plugged' activities following a use-modify-create learning progression with the Micro:bit device to practice these skills. The study used a quasi-experimental design to compare students' level of computational thinking between the program participants and a control group. The results suggest a positive effect of the learning activities on student computational thinking knowledge and skills as compared to the control group. This result persists after controlling for school context and student gender. This study provides an explicit approach to implementing these activities in the context of a developing country and assesses their effectiveness in a large-scale study.