Don't know the season, but there was an episode where the boys wonder why the girls are so crazy about Tobe Sensei. The girls tell him it's because his forehead scar is so cool, but then each one has a different version of how he got it. When the boys try to find out the true story from the other teachers and the principal, they all pull ninja escapes rather than answer. Rantarou and Kirimaru are trying to figure out what to do next when Shinbei goes right up to Tobe Sensei and asks him, and Tobe Sensei cheerfully tells them. He was fighting a duel by moonlight on a hill with a single plum tree atop it and won, but his head band was cut. The boys start to go, "Wow, that really was cool" and Tobe Sensei says, "Wrong!" His headband had been cut but not his forehead, but as he was sheathing his sword he stepped on an over ripe plum, lost his balance and fell hitting his head on a small rock. With tears in his eyes Tobe Sensei says, "It wasn't cool at all." Later the boys decide not to tell the story, then Rantarou breaks the fourth wall and tells the viewers to keep it secret too.
TL:DR Other people think Tobe Sensei's scar is cool but he finds it embarrassing.