Characters saying "Mudfart" (with an onscreen counter).
Randy drinks from a mug with the words "The Class of 1993", and since it is mentioned that Jason Myers's incident happened twenty years ago, it means that the show is currently happening in the 2012-2013 school year.
If, in the flashback, Slimovitz is supposed to be the principal of Norrisville High in 1993, Randy's story contradicts the one Howard tells in "When Howie Met Randy," in which Slimovitz first becomes a principal in 2004. It is possible, however, that Slimovitz was working as a teacher, vice-principal, or in some other capacity at the school.
The boys mentioning Mudfart's name is a reference to the 1988 film Beetlejuice.
In the flashback, Slimovitz is dressed up as the Crow from the 1989 comic The Crow.
Jason Myers dressed up as a Gameboy console, which was released in 1989.