Co-teaching is regularly paired with school improvements and educational reforms, yet research does not clearly separate the challenges of co-teaching for teacher professional development, course improvement and for wider reforms. We explored how co-teaching emerged and what barriers teachers experienced as meaningful for their co-teaching after a national core curriculum reform. Two cross-sectional data sets were collected. Three qualitatively different co-teaching profiles emerged: highly collaborative, collaborative, and imbalanced co-operative co-teaching. However, teachers’ experiences of the meaningful barriers varied. Finally, we propose a model of contextualised co-teaching that supports implementing and researching co-teaching as a part of second-order educational changes. •Traditionally, co-teaching and team teaching have been used as tools to transform educational practices.•Successful transformations require committed teachers but also support from school-, regional- and national-level actors.•Highly functional co-teaching is characterised by shared regulation of teaching practices.•A contextualised model of co-teaching supports recognition of beneficial practices at all relevant levels.