The changing landscape of demographics, technology, and diversity in the learning environment is challenging schools around the world to rethink their approaches to the implementation of high-quality teaching practices. Classroom practices are becoming more complex because educators have to ensure that their students are well-equipped with 21st century skills (e.g., Darling-Hammond, 2010; Dede, 2010; Griffin, McGaw, & Care, 2012). Educators, curriculum developers, and school administrators need to be more than experts in pedagogy. They are now required to keep up with current ideas, innovative instructional practices, and the results of a variety of educational reform efforts. Believing that teachers’ beliefs are the most important psychological construct with regard to instructional practices (Pajares, 1992) and that teachers’ beliefs are related to their choice of classroom practices and, ultimately, the students’ performance (Bybee, Taylor, Gardner, Van Scotter, Powell, Westbrook, & Landes, 2006; Staub & Stern, 2002), the author of this study utilizes the international data set of the Teaching and Learning International Study (TALIS) 2013 to examine the associations between teachers’ constructivist beliefs, their self-efficacy beliefs, professional activities, and the school principals’ instructional leadership as related to lower secondary school teachers and principals in South Korea, Finland, and Mexico. These three countries represent the high and low performers in the global index of cognitive skills and educational attainment (Pearson, 2014). An account of their educational practices will provide some insights for stakeholders in school systems across nations. Nevertheless, it is important to understand that each country has unique teaching and learning conditions, and that conclusions reached in relation to such conditions do not apply across nations. A series of hierarchical linear modeling (HLM) studies were performed for the present work to provide evidence-based information with practical implications to school administrators and educational policymakers regarding the development and implementation of leadership programs and teacher professional development. Additionally, an understanding of how the constructivist beliefs associate with the level of self-efficacy and professional activities will assist curriculum developers in higher educational institutions in the development of quality teacher preparation programs for the future teaching workforce.