The purpose of the study was to examine the influence of informal groups on school leadership practices of secondary schools in Horo Guduru Wollega Zone. To conduct the study,questionnaire and semi-structured face to face interview were conducted to collect the data. The researchers used simple random sampling technique to select respondents. The sample of the study included 8 secondary schools, 94 teachers, 23 principals, 6 Supervisors and 14 Woreda education office experts. The analyses of the data were conducted using percentage, frequency and chi-square. The finding of the study revealed that in secondary schools, informal groups were found to be formed because of poor communication channels, poor leadership skills, lack of involving followers in decision making, personal interests, lack of conducive work environment in schools and the division that leaders make among followers. In addition, individuals were found to involve in informal groups to satisfy their interests and sometimes to defend themselves from some measures may be taken against them. Furthermore, school leaders lack transparency in decision making. Finally, the finding of the study revealed that school leaders are expected to be aware, should foster good team spirit and positive relationships by turning informal groups into active organizationâ€™s participatory groups whose main objective would be to help every member to improve studentsâ€™ academic performance which enhances positive relationship amongst the teachers and that school leaders should identify themselves with the groupâ€™s informal leaders to gain corporation of the group and be skilled in school management, and get continuous professional trainings in the service they render.