Although Computational Thinking and Programming have become obligatory in many national curricula, the majority of teachers in practice are currently in dire need of support from both the research and teaching community. A national research and teacher development project was initiated in Sweden to address this issue. To develop their pedagogical competence in these new subjects, 31 in-service teachers from the project performed lesson studies at their schools. This study collected and analyzed the documentation of 12 lesson studies regarding how computational thinking skills are taught and assessed in K-9, with block-based programming. The study applies a validated CT framework to identify the involved computational thinking skills. The result presents a progression stage scheme for these skills in compulsory schools. The paper also discusses the problems and challenges in the assessment of computational thinking skills.