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.