Never Been Kissed
In the “famous couples” prom scene, a pregnant Mary and Joseph try to sit at Kirsten’s table, but she tells them there’s no room for them. That's a direct Biblical reference to there not being any room at the Inn during the birth of Jesus. A more on-the-nose Easter egg, but a fun one nonetheless.
Star Wars: The Force Awakens
While this is so far unproven, there’s a theory from the silent scene with Rey and an older Luke Skywalker that a gravestone can be seen in the background. Who does it belong to? Only true Star Wars fans could possibly have any theories.