Must I denounce the student that has scolded his teacher on your site?

Just leave it. There are many students out there who complain about their teachers. Just like employees complain about their bosses. Just like kids complain about their parents. It's stupid, but don't bother reporting this to the teacher. Honestly, if the prof knew he was being called "bad", he'd probably just laugh and shrug it off. It isn't even a proper insult.