I had a student that used to get into fights and was extremely aggressive and violent towards others, on the last day of fifth grade his last words to me and his class were "fu*k you!" -- many years later he came back to the school I'd been teaching at and looked for me so he could give me a big hug and apologize. In his words, "I was garbage when I was here, thank you for putting up with me and I'm sorry".
I cried like a baby - I was so proud of him.