Its happened more than once
I was sitting in french class beside a girl named Sandra. A very good looking girl I might add, but is known to sleep around if you know what I mean.
I was sitting in the very back of the room with Sandra. She rested her hand on my upper leg 'very upper leg' and started to rub the outside of my pants up between my legs. I was rock solid in jeans so it wasn't too noticeable. I looked at her and gave her this what are you doing look.
She wouldn't stop she just went faster. I couldn't take it anymore and just busted in my pants harder then ever. She giggled and then the bell rang, we smiled at each other and then went on.
This happens now every so often maybe next time I'll try to give something in return. I shall write again and tell you how that went!!