To involve students of a high school geometry course in the proving process, it was replicated a methodological innovation that has been implemented in a university geometry course and whose effects have been interpreted as successful. One result of this replica shows an unexpected fact by both the research group and teachers involved in the experience: there is a mismatch between students’ verbal productions when they engage in collective construction of the proof of a conjecture formulated by whole class as product of the solution of a problem, and the productions that the students autonomously perform when they must report in writing the proof that was previously constructed collectively. This problematic issue is explained by an analytical device that articulates constructs such as instructional situations, didactic contract, sociomathematical norms and reflexive discourse, from which the interaction in the classroom and the teacher’s role in it are studied.