This study investigates the role played by the Namibia National Teachers’ Union (NANTU) in school management. It examines the challenges faced by school principals in managing schools as a result of the presence of unions. In order to understand the behaviour and impact of union representatives in schools the study makes use of literature on teacher unionism, democratic, political and ambiguity management and leadership theories. Working in the interpretive orientation the study used semi-structured interviews, document analysis and focus group discussions. The research was conducted in the form of a case study involving three schools in the Kavango region of Namibia, and the data collected were dealt with according to case study principles. The study reveals that the presence of the union has several benefits for the school: for example, it advances the democratic participation of stakeholders, provides feedback to the school principal on how staff members experience his or her leadership, and offers advice on labour-related matters. It was also discovered that NANTU representatives act as mediators in conflict situations, which means that teachers can become better at managing and resolving conflict through their experience of serving on the union structures. However it was also discovered that the presence of NANTU in schools has several negative effects on schooling. For example, NANTU activities can disrupt school programmes; moreover, in some schools, neither NANTU representatives nor school principals understand their own or each other’s roles, and consequently they are always in conflict. The absence of a policy regulating the representation of NANTU in decision-making structures in schools causes the conflict to escalate. It emerged that the role of NANTU in schools is mainly determined by the leadership style of the school principal and the effectiveness of the NANTU representatives in that particular school. For example, an autocratic leadership style on the part of the school principal tends to exacerbate the conflict situation, while NANTU has been known to mobilize learners to demand the removal of school principals through class boycotts. NANTU is more concerned with defending the rights and interests of its members than any other matter affecting education. This research is likely to benefit school managers, policy makers and implementers, and NANTU structures, as it provides clarity on what both NANTU and school principals expect from one another. The study also demonstrates that school principals need to acknowledge and consider the divergent nature of the various interest groups in schools.