This mixed methods study investigates ESL instructional practices at Shenzhen Experimental High School, a Chinese international secondary institution. The findings will provide data-driven insights into optimal teaching methods for Chinese adolescent ELLs. This holds significant implications for enhancing teacher preparation and professional development programs in international schools striving to meet the needs of diverse linguistic learners at Shenzhen Experimental to identify areas for improved professional development tailored to this context. The goal is informing data-driven reforms to enhance outcomes for the schools adolescent English language learners (ELLs). This research also provides an important foundation and model for future inquiries illuminating the multidimensional competencies that enable ESL teachers to create inclusive, linguistically responsive learning environments.