It might have been fourth, or fifth grade, but I know I was still at primary school when I did this. I can still picture the classroom I was in at the time, with the old wooden desks and chairs, and me sitting towards the front of the room. My teacher at the time was Mr Cummins. I remember one day he had me sitting under the blackboard out the front for punishment for talking in class, which was kinda funny considering I had laryngitis at the time.
It isn't actually a scar, just a permanent reminder as to how stupid kids can be some time when they're bored. On this occasion, I think we were drawing, and yours truly had no doubt finished what he was doing and was wondering what else he could do. So, with a thick black crayon in my right hand, I picked up a compass with the left and started stabbing it.
After successfully pricking it three or four times, one blow missed, scraping the side of the crayon and piercing my thumb. The black spot one can just see is a bit of crayon that went in with the implement, or though some people would argue it's a blood clot. But I beg to differ.