In the discussion about having a party after the rumble to celebrate stomping the Socs, Ponyboy innocently inquires if Sodapop would bring Sandy to the party. Unbeknownst to Ponyboy, Sandy had to go and live with her grandmother in Florida. When Ponyboy still does not understand the circumstances of her departure, Steve gets angry:
"Does he have to draw you a picture? It was eigher that or get married, and her parents almost hit the roof a the idea of her marryin' a sixteen-year-old kid" (111).
Based on the evidence provided by Hinton, the reader can conclude that Sandy had to move to Florida because she was pregnant, and this scenario fits well with the historical context of the novel. Teen girls who found themselves pregnant out of wedlock were often shipped to a relative's house out of state, usually with the idea that they could discreetly have the baby there, put the child up for adoption, and then move home with no one in the community being any wiser.
Interestingly enough, Darry reveals later that Sodapop was not the father of Sandy's baby, but he still "wanted to marry her anyway, but she just left" (174).