How can we promote students to work together with mathematics? In this intervention study, groups of students in a multilingual upper secondary mathematics classroom were promoted to work together to solve mathematical problems instead of doing individual work in textbooks. Different tools were developed in a cyclic process to help students develop their reasoning and communication abilities in mathematics. In the study, focus was on the changes that occurred in the student-tostudent interactions, what caused the changes and what conclusions could be drawn about the complexity concerning interactions in multilingual mathematics classrooms.