When a plane flies over the island, the girls think they're about to be rescued. However, Gretchen later tells her staff she paid the pilot off to not say anything. We continue to see the girls on the island for weeks after, and we see their interviews in the bunker, but not the rescue in between.
It's possible the researchers rescue the girls after the shark attack, especially if another girl dies trying to save Rachel. After all, the fewer subjects the researchers have, the less research they can obtain. However, I could also see the girls being on the island for longer than 23 days; Gretchen may want them there for a solid month since we know how important the island time is to her.
The girls also seem to have limited knowledge about what's really going on, especially since Daniel claims he's there to make them feel comfortable when he's truly instilling guilt and shame. Hopefully, Leah and Shelby are able to share the information they've learned with the other girls.