lol yes I had that happen once during a dance. I just felt warmth on my crotch and she leaned against me and I was thinking "wtf" then she said "Please don't tell anyone! Come with me real quick ok?" I went with her to the girls bathroom and she locked the door. Well from there was just nature.
yeh, this happened to me at my ex gf's house once... we were hangin with some frends and once everyone left we started making out on the couch. pretty heavy, lasted for about 20 minutes. I was turned on and was wearing jeans... it was pretty embarassing. I hated it
definitely possible if you're rubbing your crotch up against his for a while. it's not really the most comfortable thing with jeans on but the fact he has a hot chick up against him can definitely make him cum