To better understand ordinary teaching practices, this article compares the way a physics teacher co-constructed reference with her students working with a half class group and with the whole class group. We observed that when the teacher works in a half class group, she succeeds conditionally in building an effective partnership with students for elaborating knowledge. We also observed that the strategies she uses are less effective in the whole class, collaboration being more uncertain. This leads the teacher to get an «overhanging» posture.