To date, no widely used, scientifically validated instruments designed for assessing secondary education students' teamwork skills exist. As CATME-B shows promising results and might suit secondary education's needs best, this study offers an initial investigation on (1) how students and teachers perceive the use of CATME-B in secondary education to assess students' teamwork skills and (2) whether CATME-B is effective in detecting changes in students' perceived teamwork skills when a group assignment is introduced. A mixed-methods in-depth approach with a smaller group of 79 students (surveys before and after a group assignment) and three teachers (semi-structured interviews) was adopted, offering a multi-actor perspective. Results revealed that students perceived CATME-B as feasible to use for self- and peer-assessment. Although teachers were more focused on practical impediments, they appreciated its content and offered valuable suggestions for using CATME-B in secondary education. Additionally, CATME-B was able to detect significant changes in students' perceived teamwork skills in the context of a group assignment.