I could see a student becoming more and more uncomfortable with the idea as the semester went on. After spending many class hours with the professor and classmates, maybe the student feels the peers aren’t nice, that the professor has made unkind comments about the student’s other assignments, that after looking at these people she doesn’t want to know their naked selves.
Many people want to do skydiving or bungie jumping adventures, but when it comes time to make the jump, they back out.