Honest question: how does this work for corporations? Does that mean a particular employee of a corporation holds the copyright, or can the corporation itself (e.g. Disney) as a legal "person" hold a copyright?

Yeah, OP should take this as a opportunity to work on assignment design and course policies for the next time. There have been plenty of times where students did something that I viewed or suspected as dishonest but didn't report them for it because I hadn't designed my assignments well or didn't have a clear policy against it.