Ms. Blank

Ms. Blank was an unseen substitute teacher for Mr. Ratburn's class, mentioned during a flashback in "Arthur's Substitute Teacher Trouble." She was unable to show up for class, and instead Miss Sweetwater was the substitute, letting the class color and have snacks.

Trivia

 * Miss Blank is likely only a joke on part of the writers, her last name being a reference to the fact that she was never actually seen on-screen at any point in the show, leaving her true existence rather ambiguous.