Fatherz wrote:GTO or Gokusen, then

?
Yankumi? She's more like the teacher that will not give up on people who everyone else would. The only times I remember her causing a revelation, were when the person was well over halfway there and she basically just had to give them the proper nudge. At the last verifiable check her students were still unmotivated, and hadn't really learned anything she tried to teach. Plus she's tied down to Shiroken High School. Unless you're imply she allows him into the Ooedo Clan, which would turn him into another of her melodramatic Yakuza buddies, Yankumi seems an odd and not particularly good choice.
A visit from Aoko Aozaki (Tsukihime) sounds more like what you're shooting for with a permutation of:
"I won't tell you to become a saint. You should just become an adult who is able to do what you believe is right. If you are able to accept what's bad and apologize just like now, you'll become an wonderful boy in 10 years."
Of course, canon Ranma more or less was already that way at the intro. As can be seen in his general behavior patterns in Volume 1. In universe I guess we can blame the nature of the contrived situation and the umptilian blows to the head with heavy objects.