Тhe article presents a comparative analysis of the perceptions of St. Petersburg teachers from a representative sampling and analysis of opinions of teachers one of the schools of St. Petersburg that has a lot of experience in pedagogical diagnostics aimed at the students self-determination. As a result of the comparative analysis it was ascertained that the teachers are aware of the necessity of pedagogical diagnostics in modern educational process. Our research confirmed the teachers awareness of the importance of daily, close to the intuitive knowledge of their students and that the existing experience of student support based on educational diagnostics, makes it necessary for the teacher to know the students problems and difficulties. Teachers lack knowledge and skills as far as the use of different sources for information about the student is concerned. Teachers do not quite understand the importance of applying qualitative and authentic methods of diagnosis, as well as the importance of taking into account the opinions of the colleagues and need to interact with them, the significance of collectively, jointly developing a mechanism and diagnostic support for all school staff. It is shown that the training in the theory and practice of pedagogical diagnosis allows teachers to eliminate many problems of pedagogical diagnostics, namely, its being labor- and time-consuming.