Oh man, easily 6th grade.
I was taking a piss and there was this hand dryer in the boys bathroom and if you looked up into it you could see this exposed coil. This was around 1996. It was only like that for a day or two and they fixed it, but I still can't believe it was like that.
Anyway, I was about to wash up and these 3 guys that HATED my ass walked into the bathroom. The all three grabbed me and pushed my hand into the coil, burning the everloving shit out of it.
I didn't tell anyone, I just kept it hidden the rest of the day in my jacket pocket. When I got home my mom saw it and freaked out, I tried to make up some excuse like I touched a hot piece of metal while waiting for my ride. She wasn't buying it. My grandfather was there, my grandmother, my little sister, they were all at my house that day.
I just remember this feeling of UTTER humiliation when I broke down sobbing and told her what had happened.