Been in 2 that I can remember. First was in 6th grade with my former best friend (friendship ended before this). It was more of a draw, though I did draw blood (left some nice scars on his face when I went straight for the eyes with my longer-than-normal fingernails).
2nd was in 9th grade with a former acquaintence. It was gym class, playing indoor kickball (this was back in 1996, before they banned kickball) and I was tying my shoe. Gym teacher said "if you aren't touching a base you are out" so I leaned forward to put my hand on the base, when said acquaintence decided to kick me in the ass. I got up, kid said "Let's not go there" or some such as I pulled my arm back. Before the fight started, I blacked out, and what felt like instantly woke up to him punching me in the face, surrounded by the other kids. Found out later that I picked him up by the neck and slammed him into the bleachers and beat his ass when he managed to trip me and get 2 shots in. Haven't been in one since.