This actually happened!
I was around 13 when this happened. I was in Middle School and caught the eye of a girl sitting on the other side of the classroom but opposite me. She had tight black trousers on and black high-heel platform ankle boots and from an earlier story you may have read, I get seriously turned on by high heel boots.
Anyway, I just kept staring at the boots mainly but also at her because she was pretty hot at the time. I could feel myself getting hard but couldn't really interrupt the teacher to go to the toilet and jack off. My hand found itself rubbing my cock through my trousers. I'm not sure whether she noticed me doing this or not but we started looking at one another and she then starting opening and closing her legs and moving her boots around. Slowly then, I started to feel more and more intense in my cock and I couldn't believe whilst staring at her and with probably 20 odd others around me, I started cumming. It was really hard not to let out a deep breath or sigh but somehow, I managed to have an orgasm! Soon after, I confirmed it by genuinely visiting the toilets and being greeted by cum in my pants but it was worth it. That little story certainly comes to mind when I come to jacking off now!