High standards are required to succeed in the state upper secondary school system and many challenges have to be met. School principals with their leadership experience and practices are key actors to ensure the achievement of these goals. The study aims to find out Estonian state- operated upper secondary school principals’ understandings of their leadership practices and supporting factors of their work. The data were collected with semi-structured interviews with eight school leaders. They were analysed with directed content analysis and leadership practices were analysed in four dimensions: setting directions; building relationships and developing people; developing the organisation to support desired practices; improving instructional programmes. An inductive approach was used to analyse data to explore the supporting factors. The findings showed that principals followed all dimensions of successful leadership practices. An important dimension appeared to be building relationships with students and teachers. The main concern in their work regarding school improvement challenges was teachers’ workload. Principals saw the strength in having trustworthy and supporting relationships with the operator and being part of the state upper secondary school network. It would be recommended to systematically assess and develop the leadership competencies of school leaders along the dimensions of successful leadership and to closely address the professionalism and support of the school operator, because it was identified by school leaders as an important factor influencing their work.