When Homer is driving through "Suicidal Moron Pass", the license plate on the front of the car is NABF14, the production code for this episode.
When Lisa walks around the abandoned class room, one wall has posters with the symbols from Avatar the Last Airbender (Water Tribe, Earth Kingdom, Air Nomads, Fire Nation)
On the blackboard in the fake classroom, the equation on the board is the same one the teacher at the genius school in "Bart the genius" season 1 used as a joke that Bart didn't understand (DY= RDRR)
When Homer flies off in the Duff blimp he shouts down to Barney: "When we see each other again, you'll be an old man and I'll be a baby," a reference to the twin Paradox in Einstein's special theory of relativity and the movie The Curious Case of Benjamin Button. It is perhaps more likely to refer to Michael Moorcock's A Nomad of the Time Streams steampunk trilogy. Homer delivers the line from a blimp/airship which features strongly in Moorcock's central character's life: 'Oswald Bastable' was in turn borrowed from E Nesbit's children's stories.
The chalkboard gag is a reference to Kristen Schaal's name being misspelled as "Kristen Schall" in the closing credits of "Homer Scissorhands".
This is the second time an episode's chalkboard gag has referred to the immediately preceding episode. The first was in "Homer the Heretic", where the chalkboard gag "I will not defame New Orleans" referred to complaints the show received about a song about New Orleans in the previous episode, "A Streetcar Named Marge".