What determines the success and effectiveness of leadership in schools? In the process of managing administrative and educational activities, the leadership effectiveness of school leaders is greatly influenced by a combined set of factors, possibly from leadership competencies and styles as well as local and global socio-economic landscape, latest advancements of science and technology, school culture, and change in modern leadership and management practices, etc. Based on extant literature review of rigorous studies on school leadership as the main methodology adoption, the present paper purposefully seeks to propose a theoretical model of school leadership effectiveness through investigating a network of relationships amongst leadership competencies, leadership styles, external and internal factors, and leadership effectiveness for global K-12 schools. Ultimately, the proposed model has both theoretical and managerial implications. Accordingly, an insight into the nature of these relationships will first expand the existing literature in school leadership and then activate school leader self-efficacy to enrich themselves with standout knowledge, skills, and qualities in the industry and simultaneously tailor their leadership styles to specific educational practices towards change and innovation to optimize leadership effectiveness.