An important educational goal for the next generation is the ability to think computationally. Cultivating K-12 students' computational thinking (CT) requires the capacity building of teachers through teacher development programs. However, if students' learning outcomes were not assessed, it is not known whether the enhancement of teacher capacity could impact on students' learning. This study aimed to examine how a teacher development program changed teachers' content knowledge and collaborative engagement, which in turn influenced their students' learning achievement. The participants were 81 teachers and 3226 students from primary schools. We employed multi-level modelling to study the relationships between teacher-level variables and student achievement assessed by a CT concepts test. The teachers' content knowledge and collaborative engagement were significantly enhanced through the program, which in turn had a significant impact on student achievement. Teachers' years of teaching experience, but not their gender or whether they had majored in computer science, could also affect student achievement. The results indicate that a teacher development program can play a significant role in enhancing the teachers' CT content knowledge and collaborative engagement, which can then have a positive effect on student learning.