i would say that you need to think about what will happen in thte future - while this system only really lists the names and addresses of students and employees, it's not unreasonable to say the following
students take classes , employees do not
employees have timesheets, students do not
employees get appraisals
students get grades
+1
if any of the above ever make it into your functionality then a "role" table is (IMHO) not the best way to go - (plus with a role table you might have real issues if you are a student and an employee - as happens at some universities)
He has already mention Many-to-Many relationship (thus association table) between Person & Role. However it will make the scenario overcomplicated with above questions.