This study evaluated a professional learning approach using a core team (CT) model to assist primary (elementary) schools to develop whole-school collaborative conflict resolution processes. Thirteen schools were matched and randomly assigned to the enhancing relationships in school communities programme ("n"?=?10) or a non-programme control group ("n"?=?3). Programme schools provided a core (professional learning) team, who attended professional learning days, and disseminated programme content throughout their schools. Programme schools also received one full school staff workshop. After one year, CT participants were more likely to apply a collaborative conflict resolution model to problem scenarios and report greater knowledge and skills compared to non-programme-school control participants. Compared to the non-programme control group, non-core team programme school staff described using more cooperative approaches to handling conflict, especially when they had received more professional development from their CT. Programme school teachers taught more hours conflict resolution curriculum, and increases in hours taught by programme (but not control) teachers were associated with teacher reported increases in student understanding and use of cooperative methods. Patterns also supported a role of self-efficacy in implementation. The potential usefulness of a CT professional learning model for assisting schools to develop cooperative conflict resolution approaches was supported. (Contains 3 tables and 1 figure.)