Nothing else is expected to be given or received. But teachers and students are engaged at multiple levels. In students' earliest years, they are forced into relationships with their teachers. A wise teacher knows that it is his or her job to meliorate this compulsion, as well as the many other discomforts of schooling in a mass society, e.g., arbitrary schedules for eating, drinking, defecating. Students soon pick up on the fact that teachers themselves are often restricted in what they can do, even if the activity is interesting and educational.
Teachers know students in certain ways better than their parents do.
Teachers understand individual students in relation to other students of the same age and background. Teachers' experience gives them a sense of comparison parents often lack. Teachers are less likely to be obsessed, as parents often are, with a student as an individual. In this respect, lawyers, who serve clients act more like parents than teachers. No less important, teachers may know better than parents what a child is capable of.