3 things.
First was way back in Grade 7 (11-12 years of age), where this douchebag had recently kicked me in the wrist, and sprained it. It was in a cloth-cast. Anyway, he was starting shit with me in one class where we had a substitute teacher, and we were watching a movie. I stand up, turn to him and say "Back off, right fucking now. You already sprained my wrist for no reason, and if you keep on messing with me, I'm going to beat the shit out of you, wrist and all." Everyone in the class was watching (and were all on my side. This kid wasn't popular, because he was a violent douche), and the teacher was trying to tell me to sit down. I ignored her completely. The other kid stood up to challenge me, I guess, so I took two steps until I was face to face with him. Then I just stared him down until he went and sat at the back of the classroom. So I said "That's what I thought." and sat back down where I was before. Everyone was like "Fuck yeah, Nick."
Second time was in Grade 9 (13-14), where some little shit of a bully who constantly gave me crap in class. Anyway, one day he was late to class, and as he was walking to his table, he knocked my books off mine with his hands. I instantly got up, grabbed him by the collar, pulled him back to where my books are (in the middle of a lesson) and told him to "pick my fucking books up, now". He said "Err, why should I, I didn't do anything?" So I threw him to the ground and told him not to fuck with me. He still did, just nowhere near as bad as before. My teacher caught up with me after and said that he agreed with me, and I wouldn't get in trouble.
Third time, today (only reason why it's memorable). Two little shits (I'd say about 14) came through my register (I'm a checkout chap), so I asked them how their day was. I'm obliged to ask, otherwise I can lose my job. I HAVE to ask. Anyway, one of them said "Oh fine. I'm, you know, doing something with my life. Unlike you." I snapped internally. Instead of yelling or anything, I just smiled and said "Oh, I'm only doing this job in my gap year to earn some money. I'm going to university next year to study Psychology. I've already been accepted, I'm just taking some time off before. Speaking of, shouldn't you be at school? That'll be 5 dollars 90." Little fucker was speechless. He paid his money and left.