This study aims to reveal school principals' unethical behaviours based on teachers' perceptions within schools, to reveal the causes for these behaviours and recommendations to prevent them. An interpretive qualitative approach was applied. Data were gathered via face-to-face interviews conducted with 17 public school teachers in Kocaeli Province of Turkey. The data was examined through content analysis technique. The participants listed the unethical behaviours of school principals in seven different themes as discrimination, favouritism, violation of the rules of courtesy, misconduct, mobbing and pressure, neglect of duty, and other behaviours. The causes of unethical behaviours of school principals were stated as the factors related to administrative qualifications, personal factors, institutional factors, the factors related to teachers, and environmental factors. To prevent unethical behaviours of school principals, the participants suggested some training activities as post-graduate education in educational administration and psychological guidance to school principals. They also suggested some activities to increase the professional development levels and human relations competences of school principals. It was also recommended to make some legal regulations regarding the appointment, assignment issues, reward, rotation, and supervision of administrators. In addition to these suggestions, it was recommended to organize social activities for preventing unethical behaviours of school principals.