20. When I Slept In Class, It Was Not To Help Leo DiCaprio (Season 22, Episode 1 - Elementary School Musical)
Here's a reference to one of the most successful and acclaimed movies of the summer that preceded this season premiere: this gag is of course referring to Christopher Nolan's superb Leonardo DiCaprio-starring sci-fi action flick Inception, which revolved largely around the nature of dreaming. Sadly the episode itself isn't a parody of the movie, though to be fair, a South Park episode covered this ground pretty damn well about a month later.