I didn't hear you investigating the truth of what the other child said before you reacted, or anything about whether a boy or a girl had made the marks (or one's self, like myself says, we use to suck on our own arms at that age, have a contest to see who could make the largest mark, or the most even). Unfortunate choice of words it seems to me for the 13 year old friend. Could be she was jealous of your daughter or wishing attention of some sort from her own parent, etc.