The subject of analysis is classroom interaction organized according to Elkonin - Davydov's theory of instruction. The method of formal semantic analysis of a dialogue has been used to describe distribution of instructional activity between the teacher and the pupils at the beginning and at the end of the primary school age. Comparison has proved that by the end of primary education all learning activity has been handed over from the teacher to the class as a learning community that desires and is capable of setting new learning cognitive tasks and finding ways of coping with them. Two factors affect primary school student's ability to understand and coordinate actions and conceptions of partners in cooperative instructional activity: 1) mastering of object and notion oriented actions whose content is being constantly reflected upon; 2) mastering of meta-object-oriented actions aimed at the partners' conceptions of the subject of their joint work. Children master those actions in the non-reflective practice of classroom discussions which lead to considerable progress in the development of reflection - emergence of the ability to understand reasons behind the thoughts and actions of another person.