This qualitative study examines the experiences of pre-service high school biology teachers in the Mekong Delta, Vietnam, focusing on the challenges and transformative learning experiences encountered during their school-based internship. It aims to explore the dichotomy between specialized biology knowledge and practical applicability in high school teaching and uncover the significance of fostering student engagement through innovative strategies and the role of mentor teachers in guiding pre-service teachers. Semi-structured interviews were used to collect data from pre-service biology teachers who completed a school-based internship in the Mekong Delta. Thematic analysis was employed to analyze the interview data. The study highlights the challenges of bridging the gap between theoretical knowledge and classroom practice, emphasizing the importance of innovative teaching strategies for student engagement. It recognizes the role of mentor teachers in guiding and enhancing the skills of pre-service teachers in creating effective learning environments. Findings suggest the need for curriculum reforms that balance specialized biology knowledge and practical pedagogical skills. The study emphasizes the importance of experiential learning through teaching apprenticeships to better prepare pre-service teachers for their professional journey.