Informal groups have a significant impact on organisations. It is seen that the researches in the literature are generally conducted on business organisations and informal groups in educational organisations and approaches to managing these groups are not sufficiently discussed. In this study, the identification of informal groups in schools, teachers' reasons for participating in these groups and administrators' management styles of these groups were examined. The study was conducted with the participation of 345 school administrators and 477 teachers working in the central districts of Ankara. The data were collected with 'Informal Groups List', 'Reasons for Participation in Informal Groups Scale' and 'Informal Groups Management Style Scale'. Arithmetic mean, standard deviation, percentage, frequency values were calculated in the analysis of the data; also t-test, Anova and Pearson Correlation Test were applied. According to the results of the study, the most common informal groups in schools are 'reference groups', 'interest groups', 'friendship groups' and 'benefit groups'. Teachers generally participate in informal groups in order to 'sense of belonging', 'making friends', 'being in solidarity' and 'share their feelings'. According to administrators' views, administrators manage informal groups in democratic, autocratic and laissez-faire styles, respectively, while according to teachers, administrators adopt democratic, laissez-faire and autocratic management styles, respectively. The findings show that there is a moderate positive relationship between administrators' styles of managing informal groups and teachers' reasons for participating. This study is expected to contribute to the literature by determining the types of informal groups in schools, examining the reasons why teachers participate in these groups, and the management styles of school administrators towards these groups.