A student carries a sign saying "Riverdale Jubillee ... 75 more years of Pep !'. Archie Comics, publisher of Archie and behind Riverdale, is currently celebrating their 75th anniversary (2017) - and "Pep Comics" was the title of their original published comic.
Surrendering her leadership of the Riverdale Vixens, Cheryl hands Veronica her HBIC T-shirt. That's an anagram for Head Bitch In Charge.
Veronica playfully tells Betty to "swear on the September issue." This is a reference to their style Bible, Vogue magazine. The September issue appears at the end of summer when the leaves are changing color, and is the breathlessly-anticipated guide for fall and winter fashion.
FP Jones is seen reading "Chilling Adventures in Sorcery #1" (a Sabrina comic) in the holding cell