This article highlights the need to understand mainstream international secondary school teachers' attitudes to and experiences of accommodating English as an Additional Language (EAL) learners, and what current collaborative relationships there are between mainstream and EAL teachers in terms of co-teaching and co-planning. The article draws on data collected from a questionnaire sent to secondary teachers at an international school in a major city in Ukraine and to other international schools that offer International Baccalaureate programmes in Eastern Europe. The questionnaire investigated English language training in education, attitudes to EAL in mainstream subjects and participants' collaboration with EAL teachers. Further follow-up qualitative data collected from a focus group in the school in Ukraine investigated the topics of competencies, responsibilities and collaboration with respect to EAL in the mainstream classroom. Building on this data, the discussion ultimately focuses on the challenges for mainstream teachers and how collaboration with EAL teachers is often confused and lacks definition in terms of current practice and ways forward. Recommendations for next steps of research are made.