The story presumably is taking place in 1955. But Betty mentions "The Twist," which wasn't released until 1960, and Veronica is reading Peyton Place, which wasn't published until 1956.
When Cheryl enters the Dark Room Tony asks her "What are you doing here Barbie?". However the Barbie doll did not come out until 1959.
About 7 minutes in, the scene pans across the rear of the cars parked for the sock hop. One has a 1955 Chevrolet license plate frame. Frames like these are products of the classic car hobby and began in the 1970s. In 1955, a 1955 Chevy was just a new car and would have no frame, a simply chrome frame, or a frame that has the dealer's name on it.