Simpsons Season 9, Episode 2: The Principal and the Pauper

When a celebration is held at Springfield Elementary to honor his 20 years as principal, Seymour Skinner is revealed to be an impostor when the real Seymour, Sgt. Seymour Skinner, shows up. The principal's real name is revealed to be Armin Tamzarian, an orphan from Capital City.