The song "Here I Go Again" by Whitesnake is played four times in the movie - twice with the words, once without, and once hummed. The band Whitesnake is also mentioned by Mitch when he references Nicole's high school jean-jacket. Frank also wears a Whitesnake t-shirt at the very end of the movie, during the credits.
Darcy's father says that, "It's hard to believe that in just seven months you'll be graduating from high school." This means that the current month would have to be November or December, yet all the trees and shrubbery in the movie are in full bloom.
What are the differences between the R-Rated version and the Unrated Version?
As many other sexual based comedies "Old School" needed to be trimmed to receive an R-Rating. 6 cuts running 79 seconds altogether are missing in said R-Rated version. A detailed comparison between both versions can be found here.