I remember having a boner in the middle of a class in high school and I had to give a powerpoint. I got up there and people started to notice almost immediately. I don't even remember why I had a boner...
For some bizarre reason, I used to always get them on any form of public transport. Must be the vibrations.
I was teaching English in an elementary school in Korea and those kids have no shame. I was explaining something at the front of the class (they were probably around third grade) when a boy yelled out in broken English, "Teacher, ...up!" and starts excitedly pointing at his friends crotch. I waved my hand at him as if to say "It doesn't matter", but he didn't care what I thought, he jumped up and grabbed his friend's junk as hard as he could and started screaming "Look! Look!". It was really messed up, everybody was howling with laughter, including the kid with the boner. They were still friends after that, too.
Almost everyone I know that has taught in Korea or Taiwan has a similar story.
I had a boner and it was gym day, so we were all wearing our jogging pants. I was also (unfortunately) wearing boxers with those loose crotch areas (mea culpa ). We had to get out bags in front because it's protocol for an assessment that we put our bags in front. So our teacher tells us to get them, so I had to walk all the way to the front of the classroom and get it. I just pretended to be really tired and shizz and crouched the whole time to hide it.
When I'm with a chick I can usually control it even if she's slobbering all over my balls, you don't want to seem over-excited or desperate. Weird to pull out a full boner when you just started foreplay. Then there's other times where I'll simply hear a story on the radio that has sexual elements and I have a raging hard on in a few seconds.