Who is Jim Prideaux from Tinker, Tailor, Soldier, Spy and what is their importance?

Jim Prideaux is a teacher. He arrives at Thurgood's school on a Friday afternoon in a thunderstorm, and he then proceeds to drive both his car and trailer into the Dip, which is a big hole. Jim is supposed to teach until the end of the term, but he stays longer than planned. The students nickname him Rhino. We later learn that Prideaux is actually an inactive British intelligence agent. His code name is Ellis.