Yes it is possible. I practice that all the time. Yes sometimes it is easier than others, and making it ROCK hard while standing is not easy in the absence of any other visual stimulation. The guy may have been aided in the fact that people were watching him and he is a closet exhibitionist.
When the teacher leaves the room as I am modeling, I can do it. What is more difficult is to get to a percentage of erection when everyone is drawing me and then stop it so it is not fully erect but hangs just right. Try to maintain that while a babe is looking at it haha. I can also make it grow when the teacher is going around the circle of students and then relax it before he or she comes around to the front again. Now that deserves a metal for control. But then again I have logged in over a thousand hours on the stand nude modeling which gave me tons of practice. There were times it didn’t work, and other times it went out of control but eventually I pretty much mastered it.
Dance