This study builds on a research-practitioner partnership embedded within an education for sustainable development (ESD) project and aims to explore the major potential challenges (i.e., disciplinary boundaries set by subject specialization, especially in secondary education) and success factors (i.e., teacher co-learning experiences in ESD) associated with differences in teachers’ professional action competence (PACesd) in a sample of 557 in-service teachers in primary and secondary schools in Flanders, Belgium. The study employed a recently validated PACesd measurement instrument and involved quantitative data analysis in a structural equation modelling framework. The results show that primary education teachers tend to report higher PACesd levels compared to their peers in secondary education. Moreover, regardless of educational level, gender and teaching experience, all teachers participating in a working group or a learning community in ESD are more likely to show higher levels of PACesd. Implications of the findings, limitations and directions for future research are discussed.