I just read this comment on a thread on Reddit and it does deliver a power lesson albeit in a rather disturbing way. Take a quick look and read the following quoted section and let me know what you think.
Junior high was OK for me. As a good student, yearbook staff member, and band nerd, I was well liked by the faculty and minimally harassed by fellow students. Skip to social studies with Mr. R. He started class one day by explaining to us that he had gone over test scores, homework, etc. and determined why our class average was so low. Quietly and determinedly, he announced that I and another boy were to blame… for everything. Then he calmly explained to the class why we were so stupid. I was so shocked. I remember staring down at my desk and crying silently while he proceeded to humiliate me and the other boy in front of the entire class. Then he told me to pick up my desk and move it to the back of the room, facing the wall. He barked at me again to move it when I squeaked out, “seriously?” So the other boy and I silently moved our desks to the back. The harassment continued. The other boy was crying, and I think I was near passing out from embarrassment and confusion – I had aced our last test. The class was incredibly quiet. My friends gave me pity looks or avoided eye contact, and one girl actually started to laugh out loud at me. We sat like that for half the class. Then Mr. R stopped his lecture and asked why nobody had stood up for us. When he got no reply, he said, “Well, this is what Hitler essentially did to the Jews. And none of you said anything to stop me from humiliating your classmates. Interesting.” My student/teacher relationship with Mr. R never fully recovered, even after he apologized.
It can be argued that the two isolated students should have known beforehand but I think it definitely delivered a more powerful because neither the two students or the rest of the class was aware of what was going on. However, depending on the students you should, and you leave that judgment with the teacher, it can certainly have unintended consequences from an otherwise very excellent example.