Résumé
My research aims at studying how school achievement is related to social background in mathematics classrooms. I am comparing two sessions held in two contrasted schools, by the same teacher on the same subject. The analyses show that the fact that some elements related to the language of mathematics are “transparent” to the teacher might explain the differentiation in students’ achievement. The discussion will show how this research, conducted in a French context, meets preoccupations of the international community of researchers, in particular about the characterization of mathematical language and the question of multilingualism.