This research aimed to examine the role of leadership in the school environment and leadership's contribution to the effectiveness of teachers' professional development in the public secondary education of Athens to improve the learning process and school quality. More specifically, this study highlights the school principal's contribution as a leader in teachers' professional development. The choice of leadership style (education, administrative, transformational, ethical, participatory, and contingent) positively impacts teachers' professional development. Data were collected from 180 teachers of the public secondary education of Athens, involving lower secondary schools, upper secondary vocational and general schools, excluding private and other types of schools. SPSS software was used to perform quantitative analysis of the collected data. The results showed no statistically significant correlation between teachers' gender, skills, characteristics, and abilities. However, it was observed that there was a statistically significant correlation between age, tolerance, and rejection of educational techniques on teaching practice. The evidence from this study confirmed a direct impact of the school principal's training on teachers' professional development. Finally, it was concluded that the school principal - the leader should work as a learning manager to lead teachers in professional development and as a learning manager and an inspiration for lifelong learning.