When I was in my last year of primary school, I got into the habit of secretly masturbating in class. When I was under pressure to finish my work and time was running out, the tension would somehow turn into sexual tension. I would furtively put my hand up the leg of my shorts and masturbate under the desk while pretending to study my work.
The feeling I made had me captured in its spell. Even if I was getting further behind with my work, I couldn't stop until I had reached that exquisite moment of release. I would then have to rush to complete my work in time. I became very addicted to this activity, and masturbated almost every day in class.
One day I was stroking myself, with my furtive wrist movement, getting faster and faster, my head looking down as though engrossed in my work. Beneath the desk my stiff penis sticking down the leg of my shorts. Almost there; When I looked up. Across the room at a desk facing mine, were about ten girls watching everything I was doing. Some of them were putting their heads beneath the desk to look, and they were whispering and giggling at me. I was mortified, but I was so close, I just didn't have the will power to stop. After a few seconds I just orgasmed in front of them all; quietly shuddering as they stared.
Afterwards I was so embarrassed and worried that it would get 'round. But it just seemed to die down. That cured me of doing it anymore in class. But I have often thought about it since then - with a bit of a thrill.