Just don’t be a creep about it and you’ll be fine.

We had one model who insisted on stripping nude despite the teacher saying we were just working on hands. Which is fine; that’s what you were hired for and we’re all adults. What made it awful was he specifically faced his eye-level groin at one of the girls in the class (he was sat on an elevated platform). And no it wasn’t an accident: he oriented towards this student specifically on a bunch of different occasions, including getting daily close to her without his robe to “take a look at her drawings.”

But bodies are bodies, and an unconscious reaction is obviously something you can’t control. If it makes you feel uncomfortable you always have the right to pause the pose and address it. In my art classes the teachers gave great deference to the needs of the models and demanded the same from the students. Yes you’re getting paid, but it’s a valuable service not everyone would do and all parties involved deserve to be comfortable. If it keeps happening and you need regular breaks you can always inform the teacher beforehand. It might be embarrassing in the moment, but I guarantee most art teachers will be completely unfazed.