Purpose: The purpose of this paper is to investigate the impacts of serious teacher misbehaviour (TMB) in schools from the perspective of headteachers, a largely un-researched area. Design/methodology/approach: Data were collected via the documentary analysis of misconduct cases from the Teaching Agency and semi-structured interviews with five headteachers who had managed serious cases. Findings: The research suggests four primary impacts of serious TMB, affecting other teachers, students, the reputation of the school and headteachers themselves. The paper concludes by suggesting a fifth impact affecting public trust in the teaching profession. Practical implications: Although rare, serious TMB can be highly damaging. Furthermore, the findings suggest that it is almost impossible to predict and so this paper suggests a "map" of the impacts helping headteachers to manage and contain it when/if the worst does happen. Originality/value: Empirical studies of the impacts of serious organisational behaviour are scarce; empirical studies of serious organisational behaviour in schools are non-existent and so this paper addresses that gap.