Similar story to that one, around the same age. There is an elevated walkway next to smooth, round metal light posts.  At the bottom of the light posts is a square conduit box. Kid in class decides to do a fireman slide down the light post from the walkway... and proceeds to remove his calf muscle from the bone.  Conduit box shaved it clean off like a filet knife.  Pretty traumatic for everyone involved and I think that kids leg was small and shriveled the rest of his life. 