OK, at first this story from Kanagawa reads quite simple:

According to the board of education, the teacher, who is in his 20s, attached the note to the student’s back as punishment after a female student complained that some boys were trying to sneak into their changing room. Without confirming the facts, the teacher accused the boy and made him wear the note which read: “I am an idiot. I tried but failed to sneak into the girls’ changing room.”

Teacher punished for pinning humiliating note on student’s back, yes, I can see why he might merit being disciplined.

However, at no point in the article is there any comment on the fact that the said teacher pulled the student up about his failure to spy on girls getting nekkid. We can only assume had he succeeded then teacher would have given him a gold star.