
Visible crew/equipment: In the scene during the American bandstand, at the end of the song when the teacher is dancing in front of the band members, the crews can be seen. The dressing and the hair doesn't match Grease era at all, and someone is sitting on a director's chair as well. (01:06:20)
Answer: I agree it plays a bit strange, but I always chalked it up to Sonny wanting Danny to win the dance competition and Sandy, although not bad, admits she's not on Danny's level, while Cha-cha is "the best dancer at St. Bernadette."
Also, he was drinking and upset so not thinking.