Well this is what happened to me a few years ago at school.
I was asked to host an assembly (as everyone in year 11 had to do at one point in the year). There was no way to avoid it. If you didn't show up on the day, then you had to do it on the day you come back. And me being rather shy was nervous about it, and to make matters worse I had to host it alongside my crush. I thought I would forget my lines and make a ass of myself, boy was I in for a bigger surprise...
I got up on the podium, and looked down to read, as doing saw I caught a eyeful of a view down my crushes shirt, being 16, my hormones raged like a jackhammer and got an erection. I thought big deal, no one will see the bulge this far away and read the short speech, then steeped down, no to applause, but a lot of laughter and wolf whistling. I stood there thinking what the hell, the rise in suicide through bullying isnt funny, when a very pissed off head teacher screamed "cover your shame boy!!".
I looked down, and to my horror my cock was in FULL VIEW, I had forgot my to do up my fly, and since I don't where underwear you get the picture.
In my haste I spun around, whipping my crush across her ass, and another wave of laughter came, I didn't wait around much longer to find out her reaction I tucked it in and ran as fast as I could out of there.
The last thing I heard was someone sing crack that whip.
That was 3 years ago, and I still get the piss taken out of me