Sustainability education has emerged as a critical pillar of 21st-century learning, particularly within international schools that serve as global hubs for fostering environmental awareness and sustainable practices. This study examines the implementation and challenges of sustainability education policies in international schools across the United Kingdom, focusing on policy frameworks, institutional adoption, curriculum integration, and barriers to effective execution. Through a policy analysis approach, this research evaluates the extent to which national education policies, international sustainability guidelines (e.g., UNESCO’s Education for Sustainable Development - ESD framework), and school-specific initiatives contribute to the integration of sustainability principles in educational settings. Findings highlight disparities in implementation, where some international schools have fully embedded sustainability in curricula, extracurricular activities, and school operations, while others struggle due to policy inconsistencies, resource constraints, lack of teacher training, and competing academic priorities. The study further explores the role of governance structures, accreditation bodies (e.g., Council of International Schools), and multi-stakeholder engagement in shaping sustainability education within international schools. Comparative case studies reveal that schools with strong policy alignment, administrative commitment, and active student participation tend to achieve more meaningful sustainability outcomes. However, systemic challenges such as financial limitations, resistance to curricular reforms, and the absence of standardized sustainability assessment metrics hinder broader policy success.