The quality of leadership exercised within the educational unit is paramount to developing an effective school and enhancing student achievement. The purpose of this paper is to investigate effective leadership in decision-making and mediation in the relationship between teachers and parents in a specific region in Greece. The method of quantitative research with sampling was chosen. The research instrument used was a specially designed questionnaire whose questions were based on other research with similar thematic content. Data entry, processing and analysis were performed using the statistical program IBMStatisticsSPSSversion 21. As for its results, there are two important characteristics that a principal must possess for the effectiveness of his/her interventions in the role of mediator. These characteristics included the participation of parents and teachers in problem solving and the individual briefing and action of the director in order to effectively mediate in the restoration of relationships. What emerged from the factor analysis conducted to highlight the main characteristics that a principal should have in the role of mediator was cooperation, organization, the use of body language, practical applications, character potential, ethics, training and information. Among others, the paper concluded that the principals who take on the role of mediator perceive that their role has a positive response and is efficient for both parents and teachers. On the contrary, deputy principals and teachers who are receivers of the role and not actors, perceive that the response is not positive and the role of the principal does not promote efficiency.