Three keys unlock the principal's office, Mr. Burn's fireplace and Krusty's Office. The key to the principal's office can be found behind the bar in Moe's Tavern. It is in the rightmost box by the trashcan. The key that unlocks Mr. Burn's fireplace is on the Stonecutter's podium (in a box). Krusty's office is opened with the key found in the middle drawer of the mayor's desk, hidden in the upper left-hand corner.