CLIL teaching and learning has attracted researchers’ attention over the last few years since research on content and language integrated learning has been conducted across different contexts, including Sweden. However, teachers’ perceived challenges with the integration of CLIL in their teaching could be further investigated, especially in classrooms containing students of different proficiency levels of English. The present study therefore examines teachers’ perceived challenges with teaching classrooms containing L1, LX or mixed proficiency level students. The study was conducted at an international, English-speaking IB school in Stockholm, Sweden. Since the school chosen for this study is located in Sweden, the inclusion of the Swedish language is also investigated since it is the official language outside the educational environment. Six teachers, 3 content and 3 language teachers, were interviewed semi-structurally. Two lessons, English as a foreign language (EFL) and physics, were observed in order to widen the discussion and compare concerning teachers’ interview answers to their teaching. The results from this study indicated that teachers would experience difficulties with teaching beginner’s levels of English and that almost all participants would make references to the Swedish language, to Sweden and its culture.