This article, based upon the field of comparative didactics, seeks to contribute to the identification of generic and specific features in the teaching and learning process. More particularly, its aim was to examine, through the study of two different school subjects: biology and English as a second language, how "passive didactic differentiation" can develop and account for the gap in progress growing between more-able and less-able students. For our analysis, we adopt a didactic viewpoint basing our study on what is going on in the class when the teacher and her students interact and use notions borrowed from the Joint Action Theory in Didactics. At the end of the article, we mainly argue that more teacher training focused on "objects of learning" and "knowledge-in-use" is necessary if we want teachers to be able to produce didactic milieus adapted to students with mixed abilities and, more generally, if we want to increase "epistemic access."