Validating a Teacher Professional Development Program to Develop Leadership and Mentorship among in-Service Teachers

School teachers are pivotal in shaping the world's future. Their ongoing professional development (PD) is crucial to fostering a positive school culture. This study aimed to develop and validate a PD program for in-service teachers, ensuring its content and empirical validity in enhancing teacher leadership and mentorship. The module underwent content validation by 14 experts using Lawshe's content validity ratio and the content validity index to determine its appropriateness and alignment with PD proposed by the National Education Policy 2020. Empirical validation was conducted with 30 participants using a single-group pre-test–post-test design to measure the module's impact. Content validation confirmed the high expert agreement, indicating strong content relevance. Empirical validation revealed significant improvements in participants’ leadership and mentorship competencies as evidenced by positive post-test outcomes. The findings suggest that the module effectively enhances teacher professional development (TPD) by improving association, professional learning, assessment, instruction, community and policy in leadership, and self-efficacy and outcome expectancy in mentorship. The validated module is a structured framework for TPD that contributes to evidence-based educational training. Future research could explore its long-term effectiveness with larger samples and a mixed-methods design, employing multiple models and frameworks, including India-specific ones, across diverse educational contexts.