Purpose: The purpose of this paper is to investigate the way informal groups rise and operate in the Teacher Association in a Secondary Vocational School Unit in Greece. More specifically, the way the role of the head teacher, the school culture and teachers' emotional intelligence impacts these groups is investigated. Design/methodology/approach: Qualitative research through in-depth interviews with teachers and the head teacher as well as the researchers' participatory observation has been conducted, in order to support the selected method of the case-study. Findings: The findings showed how both positive and negative informal groups rise and function in the Teacher Association. The role of the head teacher emerges as a very significant factor that influences the emergence and the preservation of such groups. The school culture has a bidirectional relation with the existence and quality of informal groups. Emotional intelligence also plays an important role in forming informal groups and in the quality of actions of these groups. Originality/value: This study covers a significant gap in the international literature of group dynamics in a Teacher Association and provides practitioners with valuable insights regarding the underexamined factors that lead to the formation, operation and preservation of informal groups, the study of which can lead to the development of sophisticated scales of measurement of these dynamics by future researchers.