This article offers an analytical proposal for the knowledge construction of the classroom discourses based on the model of language as a social semiotic offered by Systemic Functional Language studies. The article presents tools to distinguish a unit of analysis from the classroom discourse that is consistent and theoretically fundament (learning activity), as well as analytical tools of the knowledge construction based on the ideational metafunction. This proposal allows to explore how knowledge is constructed in different instances of classroom discourse, as well as to study different aspects of the way in which knowledge is constructed in pedagogical interaction.