Abstract: Quality factors of learning process in the formal educational institution depend on the school principal leadership, cultural organization and teacher’s competence. Education quality is a partial achivement of educational goals and competence of graduates which have been assigned to the appropriate strategic plans in achieving those goals. This study involves input aspects, process, output as well as value and the degree of goodness, righteousness and perfection. The study was conducted in Indonesia private High School Brigjen Katamso I in Medan to see the dominant factors affecting the quality of school, the school leadership, school’s culture and teacher competence. The study population was all high school student (756 student) with sample of 255. The method used is a survey method with a quantitative approach. The analysis result showed that the effect of principal leadership, school culture, teacher competence towards school quality is respectively 0.3893, 0, 289 and 0, 347. Overall, the coefficient of determination (R2) is approximately 0.841. In other words 84,1% of the variance of school quality can be desribed through principal leadership, school culture and teacher competence.