Student works that are part of course assignments at the University of Minnesota are unquestionably owned by the students who create them. This is true both as a general matter of law and of University policy.
Instructors can require students to share works in certain ways - this is best done by making all expectations for sharing clear in the course syllabus. A student who prefers not to share her work in that way can discuss alternate options with the instructor early on, or opt out of the course.
However, where instructors do not make sharing expectations clear up front, the instructor does not have the right to make sharing decisions unilaterally. It is always a good idea for instructors to model the respect for authors and creators that they expect from their students.