I was 18 in my last year at boarding school in the UK. It was a mixed school (girls and boys). We had just had our end of the year dance and I had hooked up with Mary-Anne, a girl a year younger then me. I had been very shy at school and had not had much experience with girls other than a few "fumbles".
At the end of the dance I guided Mary-Anne to the corridor at the foot of the stairs to the boys dormitories. There was nobody around so we kissed, carefully at fist and then with more passion, my first French kiss! I let my hand move up her body outside her blouse and onto her breast, then I moved under her blouse and up to her breast again, I pulled down the material of her bra and rubbed her young nipple between my finger and thumb - I heard her moan softly.
I was so engrossed in what I was doing that I hadn't noticed her tugging at the zip on my trousers, I was hard as rock and it was with a shock I realized her hand was in my pants and holding my penis through my boxers. I held my breath as she pulled my cock through the fly hole and gripped it firmly. I later found it was the first penis she had touched.
I was in heaven - a nipple in my hand and her hand around my cock. We broke off the kiss and looked at each other - a small nod from me and she pulled my foreskin up and over the head of my penis. One more stroke back and mortified I realized I was going to come. Without a chance to stop her or even give a warning hot spurts of semen were filling my pants and her hand. She froze for a moment then smiled at me - suddenly pumping fast until I could take no more and begged her to stop.
Next day the whole school knew I had blown my load in my pants. Shame of it - but I never forgot that first hand job.