South Park (1997)

S2E7 City on the Edge of Forever (Flashbacks)

As the school bus teeters precariously over a cliff, the boys reflect on their previous escapades. Meanwhile, Mrs. Crabtree hitches a ride with a stranger and unexpectedly finds herself performing stand-up comedy for a short time. Back in South Park, Mr. Mackey persuades the parents that their missing kids have simply run away. Ultimately, it all turns out to be a dream layered within another dream.