I was eleven when this happened in my last year at junior school
I left my junior school in England around 1981. We had a teacher in those days who made you do PE (that's gym) lessons in your underwear if you forgot your kit. (In those days this was usual, wouldn't happen now). One day I forgot my kit so I did the lesson in just my underpants. We had the apparatus out for climbing on. I felt kinda nice and I know I had an erection but I don't think anyone noticed. I sort of got a kick running around the gym in just my underpants. When I climbed the rope it felt really good and the rope rubbed me as I climbed it. After climbing it about four times I felt the feeling come over me as I was sliding down and my dick jerked about. There was no ejaculation as I was too young but it was a dry orgasm. It was awesome so during some of the next few lessons I recreated this scenario.
I did feel strange and never told anyone about this and have often wondered if my experience was unusual.