This paper sets out to clarify how educational relationships are understood in Niklas Luhmann's system theory. In this connection, special attention has to be paid to how Luhmann modified his theory. In his early works, Luhmann set up a thesis saying that "the educator is not the teacher as an individual but the interactive process called a lesson". In other words, education is not a matter of individual teachers but of educational relationships themselves. The teacher's approach is therefore understood not as a one-sided "intervention" directed at the student but as "interpenetration" between the education system and the personal system. However, if we conceive of education as "interpenetration", it became impossible to distinguish between socialization and intentional education. In this situation, following the introduction of the concept of "autopoiesis" in the 1980s, it became possible to distinguish between interpenetration as socialization and education as a communication system. Then, understanding in a lesson signifies understanding mediated by understanding of the communication system. That is to say, the student's understanding is made possible not by understanding the educator as an individual but by understanding the communication system constructed by the student and the educator. According to Luhmann's theory, the mechanism of education is uncertain simply because of the involvement of a psychic system as an autopoiesis system. However, education becomes possible at the point when a psychic system participates actively in the education system. That said, the education system will always be a problematical system. Because it cannot depend on the inner aspect or personality of an individual, it becomes routinized and differentiates people according to whether it grasps them or not. Or it may cause an unintended educational effect as a "hidden curriculum".