An old friend thinks we're laughing at his dirty humour, when we're actually laughing at who is listening behind him.

This happened about a year or two ago, when I was still at my comprehensive/high school. Me and my group of friends were standing outside during our morning break before 3rd period, and this kid called Shaun appears. We all got on with him fairly well, but we weren't really close friends with him or anything.

Anyway, because he was one of those kids that loved to drop some top quality banter every now and then, he decided the best way to make us all think of him as a "proper legend", was to shake up a can of pop (I think it was Tango, or something, I don't know), and then open it so it would fizz everywhere. :/

So, as he begins to shake the can vigorously, he eventually realises that he is making a handjob gesture. For some reason, he sees this as an opportunity to really show us all his comedic genius. Now that the soda has been shaken up good and proper, he pops the tab, and uncomfortably loud, while continuing to jerk off the can he shouts "OH YEAH BABY, I'M CUMMING!! OH YEAHHHHH".

As this point, pop/soda is flying all over the place, soaking his hands slightly, and obviously spilling onto the grass. As we all laugh hysterically, he looks so proud of himself... His dreams of being a comedian were finally coming true... He shouts "I'M CUMMING BABY!!" a little more, then stops shaking the half-empty can, to admire his still-laughing subjects. But then he realises that we're all looking past him. We could see the look of pride on his face, change to that "oh shit" expression, as he realises that something is not right. He turns around to see the fucking teacher, standing about 3 metres away from him, glaring at him. She gives him that "get your ass over here" hand gesture, as we all laugh at him and cringe simultaneously.

TL;DR: Friend shakes can of soda all over and shouts "I'M CUMMING BABY!!", while a teacher was standing behind him the entire time.