Say I have a class User and an interface Administrator. Some users are administrator and can thus implement the interface. Most aren't, however, so it's false to say that User implements Administrator.How can I represent that in a class diagram? Should I use the classic implementation arrow, or is there an existing stereotype for a class that can implement an interface?
How to indicate that a class might implement an interface?
uml;class diagram
What you want is a Decorator.An Administrator would be a subclass of User, and its primary member attribute would be a User object.Thus a Student Adminstrator would be an Administrator containing a Student (which is a subclass of User, right?  The Administrator-specific attributes are set in the Adminstrator class, with the User methods passed through from the User object.