I was in the 2nd grade, it was near the end of the year. Many of the teachers where inside the school for a going away party for the students while a few remained outside to watch the kids that wanted to play. I was a little young to remember this completely but I guess some of the teachers didn't feel like standing around to watch children play. This left a large area outside of the school with no one watching it.
I came outside and was walking in that direction and rounded a corner(not knowing there was no teachers). Somehow a rock fight (imagine a food fight with rocks) broke out. It was ending around the same time that I was rounding the corner and I watched as a child ran past me with the side of its skull missing. Blood and bits of matter was on the ground. The child ran around the corner that I just passed and I could hear the teachers start to yell.
Maybe not the worst thing to see but it really haunted me. Btw, the kid lived and is still doing fine. (big metal plate and some skin graphing.