In this paper we aim to establish a link between two theoretical frames: modelling and its use in the design and analysis of scientific teaching sequences, and the communicative approaches as they alternate in classroom activities. In this case study, we follow the interactions between the teacher and a pair of students during an entire teaching sequence in Optics (grade 11). We focus on the way the teacher managed the dialogicity and the modelling processes in the classroom discourse. A qualitative analysis shows some difficulties in such an achievement, and their consequences on students' meaning making. (Contains 4 figures, 5 tables, and 1 note.)