Well it's supposed to be a chance for artistic experience, not a chance to see a guy's penis. If I had a nude female model in my art class, I would actually draw her and not get all hot over her girlparts. I doubt anyone will think you're gay for drawing him, at least nobody who is mature enough for you to care what they think. I think they WILL suspect you're gay if you get a hard on and everybody finds out, or if you just draw his penis like you say you might do.

So I think if you're going to go for the artistic experience, go. But if you're going just to oogle his cock, just go jack off to some porn!