I can give a different side to this question from the model’s perspective, and help bring some sense here on your end.
I modeled once for a life drawing class at a local community college while I was in college my first time around. I was wearing a leotard, and sandals as I stood on the desk in the front of the class. I was given a pose, and held it for hour-long intervals.
My mother was with me, and she was allowed to draw me as well. What was cool was she was allowed to take one of the better sketches home from one of the students who had drawn more than one picture of me.
What I was appalled by was I was allowed to take a break and walk around to see the drawings done of me, and this older male student drew me naked, and in what he believed was full detail. My mother was shocked he had drawn me naked. Sure, my perfect, young, female figure must have inspired him to draw in such a manner, but I felt THAT would have been considered pornographic. However, he was not told to re-draw the picture, he was actually praised, which creeped me out.
Now, if I was the model, male, and nervously erect, yeah, I would let that slide, because well, it’s a natural reaction, especially if the guy is shy and doesn’t do those sorts of things for a living. Colleges tend to put out feelers on campuses, or word of mouth, which is how I found out about it-through my mother.
So, I would agree that your teacher is just a bit too uncomfortable with his sexuality if he’s worried about his student drawing an erection when there actually was one there. If need be, you could present your case to the dean if the teacher presses you.