This paper presents findings from a survey among Norwegian K-12 teachers (n = 771), addressing their perceived collaboration with parents when students exhibit internalizing or externalizing behaviour. Positive teacher-parent collaboration is important to enhance student learning. However, behaviour difficulties among students may put the teacher-parent relationship under strain and affect the teachers' sense of self-efficacy. Through factor analysis, analysis of variance and multiple regression analysis, the paper identifies how individual teacher variables such as gender, years of experience and additional training in psychology or special needs, and school variables such as grade levels, the schools' participation in mental health training programmes and collective teacher efficacy, may affect perceived quality of collaboration. The findings showed that collective teacher efficacy is significant for positive teacher-parent collaboration, and that teachers perceived the collaboration as more conflictual when students show externalizing behaviour. Female teachers and teachers at grade levels 11-13 also reported less negative collaboration.