This is the last episode of its season and someone (perhaps the set decorators) decided to have a little fun with the chalkboard assignments in Miss Landers' classroom. One is "Geography: Name every lake in North Dakota".
This is the last episode in which Miss Landers appears as Beaver's teacher. She does make one more appearance in the series, as Beaver's former teacher in Nobody Loves Me (1962).
Hugh Beaumont lived in Minnesota during the summers and often left the season's filming early to travel there. His brief appearance at the beginning of this episode reflects this.