Joey Richter (Apple #8) played Ron Weasley in the YouTube famous musicals "A Very Potter Musical", "A Very Potter Sequel", and "A Very Potter Senior Year", in which also starred Darren Criss (Blaine) who played Harry Potter.
Ryder (Blake Jenner) sings "I Only Have Eyes For You" by The Flamingos. This song is also featured prominently in a season 2 episode entitled "I Only Have Eyes For You" of "Buffy the Vampire Slayer" (1997), which was also about a Sadie Hawkins dance.
Joey Richter [Apple #8] and Curt Mega [Nick] both starred in the musical Spies Are Forever. And have both worked with Darren Criss [Blaine Anderson].
The arrangement of "Baby Got Back" used in this episode was in fact created by singer/songwriter Jonathan Coulton, however he was never asked permission or given credit by Glee or its parent company, Sony (in the original airing they even went so far as to use the modified lyrics where Coulton refers to himself, "Johnny C.", by name). Coulton declined to take legal action but instead listed his original single of "Baby Got Back" on iTunes, subtitled "In the style of 'Glee'", with a photo of himself using the classic "L"-to-forehead hand-gesture, which due to the resultant fan outrage over the use of his song without permission, quickly climbed to #1 on the iTunes Store rankings.