Answer: Cha-Cha
During the dance contest, Sandy and Danny got separated in the confusion of people on the dance floor and he ended up dancing with Cha-Cha DiGregorio (the self-described best dancer at St. Bernadette's), who he said was "just a girl I know, that's all", implying that they'd previously been together. Later, Danny told Sandy that he and Cha-Cha "didn't go together, we just went together, that's all." Danny and Cha-Cha won the contest, dancing to the song "Hand Jive". They then had to dance a spotlight dance, to "Blue Moon". Cha-Cha had originally come to the dance as Kenickie's date. Even though Sandy was upset that Danny won the contest with someone else, she forgave him.