In this case study, we investigated teachers' professional learning within a multinational project in an upper secondary school. The aim of the study was to investigate how the participating teachers adopted and applied the trialogical approach (TLA) in their pedagogical practices and their challenges in doing that. The mixed method approach was used for data collection and analysis. About one-fourth of the teachers participated in the activities, ten females and three males. Three groups were identified, based on their activity in the project: pilot teachers, active adopters, and adopters. Altogether 79 students (38 males and 41 females) answered a questionnaire concerning the pedagogical practices. The pedagogical revisions were well in line with TLA; the revised courses as well as new iterations and new ideas were indicators of the teachers' creative implementation processes. However, some of the TLA ideas were more difficult to apply in an upper secondary school context; for example, the implementation of ideas involving crossfertilization with other organizations and cultures was rare. In order to learn new pedagogical practices, teachers need organized time for collaborative planning, for reflecting, and for sharing.