After viewing the rest of your gallery it looks to me like you are fully engorged, even if not erect in the first pic. I notice several other pics in your gallery where you are flaccid, and your penis is much smaller, and even some fully hard where you appear smaller. The difference is so noticeable I wonder if you might have altered the image in some way, though I will assume you did not. At any rate, you can't have overlooked that, so it seems intentional that you would pose and choose this image to show off a bit, not that there's anything wrong with that. I wonder if you are seeking approval here to do that, not that there's anything wrong with that either. It's the fullness and the size of your penis that would push that subject past the point of being simply a nude study to being an erotic drawing, not that there's anything wrong with that either, unless it violates the intent of the assignment.
I actually find the seated pose less blatant, though with the partial erection, it too moves into the realm of the erotic. I'm speaking from my experience as an artist and a former life model. I never posed fully nude for classes, only partially nude and for portraiture, but I did pose nude for a few professional artists regularly in private sessions. In both situations there was a certain ego gratification that came from seeing myself well rendered from a variety of individual perspectives. One artist in particular did more erotic studies of me as part of his work. A few I gave as gifts, a few I have in my own collection, and I'm very glad I do. I would say that if your intention is to display yourself in an erotic manner, just be honest and straightforward about that, and be prepared for the responses.
[Edit: In my mind 'erotic' is different from 'pornographic', though I recognize for some that may be a fuzzy line.]