When students actively listen and interact with each other, their opportunities for developing reasoning and communication abilities in mathematics increase (Sjöblom, 2015). However, they are not always given opportunities to work dialogically during mathematics lessons, and there is little research in upper secondary classrooms about how teachers can promote student-to-student interaction. This poster presents an educational design research project (McKenney & Reeves, 2012), in which the intervention focuses on teachers’ work to make multilingual students interact while working with mathematical problem solving. The design and analysis build on the inquiry co-operation (IC) model (Alrø & Skovsmose, 2004) and Fuentes’ (2009) framework on student communication. The aim of the intervention is to find ways for teachers to promote and support student interaction, so that students engage in all dialogic acts in the IC-model, and hence listen actively to each other and ask/answer questions. It is expected that the design will need to take into consideration how students perceive group work, how they understand the purpose of questioning and listening, what role language and/or multilingualism plays in interaction, and how teachers can cooperate with other teachers to develop their teaching. Sjöblom (2015) provided results on these issues concerning the students; so now teachers are in focus. The poster presentation has the aim of discussing insights on: 1) What kind of language do students need in order to participate in fruitful student-to-student interaction, and what do teachers need to promote this? 2) How to design tasks and support means for students, such as problem-solving strategies or communicative roles, in order to make students listen and ask mathematical questions? 3) What can other possible frameworks offer to continue the investigation?