What is Holden's final goodbye to Pency Prep?
Holden yells, "Sleep tight, ya morons!" as he leaves Pency Prep on his journey to New York City.
The phrase is a sarcastic, passive-aggressive taunt. It's a cowardly act, since no one is really there to here it. And it's a cry for help: Holden feels betrayed not only by his friends (Ackley, Stradlater), but by the adults (Mr. Spencer), and even Jane (who's dating Stradlater). All in all, Holden signals his distaste for the materialistic and phony culture of the all-boys prep school.
Psychologically, the "sleep tight" is veiled reference to death: he wants them all to die in their sleep. Certainly Holden is preoccupied with death, and his expulsion send him on a collision course with near-suicide. Two deaths in particular trouble Holden: Allie's (his brother) and James Castle (who jumped from a building at Pency and killed himself). The latter, especially, troubles Holden, because the other boys viewed it callously (they "rubbernecked").
After his argument with Stradlater Holden decides he should leave the school early and go to a journey in New York before actually going back home on wednesday. He packs his bags as Stradlater sleeps and goes down the stairs. Once at the bottom of the stairs he yells, "Sleep tight, ya morons!" on the top of his lungs and leaves.