Sorry to bump in, but here are some ideas on the top of my head.
Maybe have Sarah switch phones with Karyn and plead innocence when asked if she has Karyn's. Could have Sarah's spotify on there and her contacts with cheerleaders and Biff instead of Jon.
Karyn could also lose her backpack and Sarah provides her with a spare on the way home that's super girly.
It also might not all be just switching. Maybe Sarah takes Karyn to see the movies, but a superhero movie Karyn wanted to see was sold out. So they see a Romantic Comedy or even a horror movie that makes Karyn feel more vulnerable.
