The present study evaluates the impact of the fine-tuning medium of instruction (MOI) policy in Hong Kong in the early stages of its implementation. It explores the key stakeholders' perspectives on a school-based policy via a case study, which gathered multiple sources of qualitative data (i.e. focus groups/interviews, open-ended questionnaires, school documents and lesson observations). The findings reveal that the increased amount of English-medium teaching encouraged by the new policy has led to the emergence of numerous controversial pedagogical and practical issues, many of which resemble those also occurring in bilingual education programmes worldwide. The evidence suggests that the school-based MOI policy and practices have only benefited the more capable students but sacrificed the interests and learning needs of less able students. The paper, therefore, highlights the need to inform local teachers and administrators on how to confront these difficulties based on their own experience so as to maximise the effectiveness and learning outcomes of the majority of the students.