Inheritance is not symmetric. The GradStudent has properties not shared with Student (i.e. the advisor). If we make Student a sub-class of the GradStudent, it will inherit the advisor field along with its setter and getter methods which are not meant to be part of a Student object.

PS. I know undergraduate students may have advisors. Still, I've made the simplifying assumption that they don't, at least not in a sense a graduate student has a thesis advisor.