The purpose of this study was to examine the achievement of students in math and science subjects as the impact of using English as a medium of instruction at an international standard school. A questionnaire was used as a research instrument to 190 students at one international standard school in Jambi Province, Indonesia. A focus group discussion (FGD) approach was undertaken to validate and verify the data gathered through the questionnaire and clarify some issues raised in the questionnaire. Data were analyzed by using descriptive statistics and Pearson Product Moment Correlation. It was found that the students’ demographic profile, attitude toward English and grades in math and science subjects were significantly related with their academic achievement. However, students’ perception on methods and techniques was not significantly related with their academic achievement in English, math, and the science subjects. The result showed that the implementation of English as a medium of instruction was not done well in the international standard school. This is perhaps due to the difficulty of learning science and math in English. This study provided information for policy makers, school leaders, researchers, and teacher educators to understand how the policy is implemented at the school level. The challenges of attempting too ambitious linguistic and academic goals in the school were discussed as were policy implications and future research.