This article explores the extent to which the medium of instruction (MOI) policy in Hong Kong affect ethnic minority students' Chinese language learning in school amidst postcolonial policy changes, and how the "flexible" MOI policy affects Chinese as a second language (CSL) frontline teaching in mainstream secondary schools with reference to the three orientations to language planning, namely language-as-problem, language-as-right, and language-as-resource (Ruiz 1984). It reports on a large-scale longitudinal study on the implementation of the "flexible" MOI policy, which is targeted to address the challenges that ethnic minority students and Chinese language teachers are faced with in CSL learning and teaching. Twenty-six CSL teachers and twenty students were target-sampled from fourteen schools involved in the same study. Based on an analysis of interview data through triangulation and critical discourse analysis, the authors argue that (1) difficulties in adapting to the new MOI for the Chinese Language subject would likely lead to low motivation levels and adversely affect CSL development among the students; (2) CSL teachers also experience difficulties in helping ethnic minority students to make successful transition for reasons including huge learning differences, limited time and resources, as well as marked linguistic differences between the two Chinese language variants. The authors recommend that policy reviews be carried out to enhance the quality of CSL teaching and learning in the midst of the current attempt at language standardization.