In recent years, the Asia-Pacific region has experienced unprecedented expansion in the adoption of the International Baccalaureate (IB) program. The IB Middle Years Program (MYP) features interdisciplinary curriculum design, concept-based curriculum development, and multiple assessments, which significantly differ from the traditional structures of junior high school curriculum in Taiwan. This study investigates the experiential and challenging aspects of the transformation of a traditional public junior high school in Taiwan into an IB school. The study employs multiple data collection methods, including document analysis, participant observation, and interviews. The study findings are as follows. The agency of bottom-up school transformation stems from the awareness of teachers of the exigency of the school for survival, adherence to the educational philosophy of IB, and the politically correct demand for educational equity. Moreover, the challenges encountered during the transformation process include changing the mindset of teachers about educational philosophy, aligning the IBMYP with the national education system, restructuring the organization, and redistributing tasks with limited faculty. At last, by implementing the IBMYP, teachers can apply innovative pedagogies and actively adapt to the two distinct education systems regarding educational progression, curriculum organization, teaching approaches, and assessment methods. These changes foster continuous professional development and catalyze a transformative momentum for teachers. These findings pose implications for other public schools that are considering the implementation of IBMYP.