In this proposal, we try to show how the Joint Action Theory in Didactics (Sensevy and Mercier, 2007) and linked methodologies (Leutenegger, 2000) allow us to describe finely professors’ and students’ actions, to describe the distance between expected and concrete learning, as well as the real knowledge stake of the didactic situation. The aim of this research is to show how our analysis tools bring to light some elements of the practical epistemology of two professors within two scientific disciplines. The question is not whether learning has occured, but what directions learning takes in relation to the actions used by both the students and the teacher, to deal with the events in the classroom. With the Joint Action Theory in Didactics (ibid.), the session in classroom is seen as a succession of “learning games”. These games are the expression of a didactic contract (Brousseau, on 1998) in a more or less specific “milieu” (Sensevy, on 2007), represented by a set of material and symbolic “objects” which crystallize elements of knowledge (Sensevy, 2009). The analysis of the “didactic transactions” (Marlot, 2008) at a micro level, points out the evolution of contracts and “milieu” for the studied period. It is then possible to identify what makes sign for the teacher i.e the “objects” to which the professor pays his attention (Baxandall, 1985) to regulate the ongoing students’ actions. Thus, we can identify some of the "actions control levers" of these teachers which, crossed with some elements of their speeches, allow us to reach their “practical epistemology”. So, our work try as well as to contribuate methodologically with an approach that allows student’s learning in science to be directly studied in authentic school context.

Our study focuses on two scientific disciplines in two educational « institution » (Douglas, 1999) : biology in a whole class and mathematics in a group of « inefficient students ». The two sessions are videotaped and transcripted and the teachers are interviewed before and after the lesson. In biology the analysed episod takes place in a first grade classroom, during a phase of experimentation. It describes the implementation of an in-the-light earthworms behaviour observing device. Which device having previously been designed by the students. In mathematics the episod also takes place in a first grade and concerns few students getting confused with the numeracy. The didactic situation concerns numbers composition and decomposition.

Finally, in the two cases, the didactic situation has partially failed. In both analysed episods, we observed a reorganization of the « milieu » - a kind of mutual adjustment – by the teacher and the students. This « adjustment » leads to a weakening of the original didatic stake. The socio-constructivist epistemologies inside wich the teachers implicitly recognize themselves, as well as the « official support », bear some responsibility for this « fading knowledge ». It seems important to enable teachers to objectify their own « practical epistemology », revealed during the encounter with the learning-teaching situation . This objectivation could help to give the teachers greater control of variables, in « on going » situation. The perspective is now to develop teachers/researchers cooperative research devices, wich would concern both the teacher training and the research.