The urgency of the problem stated in the article stems from the fact that in modern conditions the study of the psycho-diagnostic component of the teacher's activities is relevant in practical terms, since the functions of these activities affect the efficiency of pedagogical activity and the educational process itself, including the effectiveness of students' learning activities. The purpose of the article is to examine the role of teacher's psycho-diagnostic activity in the school educational system. The leading method of studying the problem is an empirical method of research (surveys, testing, content analysis), methods of mathematical and statistical processing, allowing a complex consideration of teachers' diagnostic activity characteristics. The study involves 100 teachers and 420 secondary school students, 14-15 years of age. The article reveals that the psycho-diagnostics, carried out by teachers, is characterized by haphazardness, and does not rely on the psychological and pedagogical technologies. The teachers carry out activity diagnostics more successfully, identifying its goals and methods. Psycho-pedagogical technologies of forming an ability to carry out personal diagnostics improve teachers' overall psychological culture. The study deepens the understanding of the psycho-diagnostic component, contributes to the development of the teacher's professional activity, expands teacher's psycho-diagnostic opportunities, and determines the levels of the teaching staff's psychological culture development. The practical significance of the study results is in their direct use for improving teachers' psychological training and other kinds of professional training in the "man-to-man" sphere.