Inspired by their goal for a well-rounded education in a world that has become more globalized, an increasing number of schools in the United States, Europe, and other parts of the world have been adapting the philosophy and curricula of international schools. While there have been several studies to support Diploma Program as an established curriculum at the senior high school level, there has been little evidence that would support any particular curriculum at the junior high school level. The purpose of this study was to investigate the relationship between external examination scores of International General Certificate of Secondary Education (IGCSE) and those of Diploma Program. A purposive sample was drawn from high school students who attended an international school in a major metropolitan city in Turkey. Data were analyzed with multiple regression approach. Statistically significant and relatively strong relationships were found between external examination scores, both in mathematics and science.