I really think more information is needed to make any reasonable judgment here. If this is a low-level course for non-Art majors (which it probably isn’t, since the OP notes that the student is at some advanced stage of study), then all of these moralizing comments about how bad and wrong it is to push students into confronting nudity may hold some water. I disagree with it, but in the spirit of the OP’s question (“Is it legal?”) I’d have to say, “Maybe not.”
But it seems to me manifestly clear that if we’re talking about any kind of truly advanced course in Art, the professor may ask the students to do some things (not all things, of course) that they don’t like doing, that disgust them, that offend them, that they object to, etc. and that such challenges may be (in this professor’s mind, and in the court’s) perfectly legal.
Let me illustrate: when I taught a course in journalism a few decades back, one assignment was for students to write their own obituaries. Obit-writing is a standard assignment (every journalist should know how to write one, and often obits are assigned to the newest hires on a paper). Some students didn’t like the assignment–they claimed it was creepy. I answered, “Why? You are going to die someday, right? Do you think you’re not? Give yourself the most pleasant way of dying you can think of, speculate how you’re not going to die until you’re 105 years old [they were allowed to include future details if they wanted to have themselves dying at future dates] and have it in by Tuesday evening.”
One student said she wouldn’t write it. It creeped her out too much. I said, “Fine.” She asked what she could write instead. I said “Nothing. You get a zero on this assignment.” She complained to my chairman, who asked me why I refused to give her a makeup assignment. I explained that, not only was the obit assignment an important skill to have in any journalist’s repetoire, it was even more important that reporters don’t apply moral scruples to covering stories. It was a bad habit to get into for a beginning reporter to say that an assignment creeped her out, or bored her, or was over her head. Pull that crap on any paper I ever worked on, and you’re not long for the job. The lesson I was teaching was “Suck it up. You dislike the assignment? Fine. Do it anyway.” I stood my ground, the student took the zero (and ended up failing my course, though not for this one small demerit) and now she teaches in my department and thanks me for helping her learn something important about doing what you’re asked to do.
If this is a true Art course, where students are being taught more fundamental things than how to draw flowers or hands, both important skills, and are taught “What is Art?”, “Is Art always about Beauty?”, “Is Art ever Disturbing or Painful?” andd other truly vital questions, then it’s important that students get creeped out and confronted by these issues.