That's exceptionally creepy. I thought they were dolls at first. But is it the fact that they do this at all (perhaps as a one-off) that's the problem, or the fact that they seem to exist on a circuit of such events, going from one event to another and losing their childhood in the process?
Before I arrived at a job in a Japanese junior high school, my female predecessor told me that the girl students (aged 12-15) were made to show their underwear to the sports teachers (male and female) so they could check that they were wearing proper uniform knickers. Apparently the practice was stopped during her time in the school, though she never found out how or why.