Originally Selena Gomez was cast as Rosie and Demi Lovato was cast as Carter. Later they decided to switch their parts and they were in fact both happier with the final casting.
When Rosie (Demi Lovato) is learning how to blend in by belching, the soda they're drinking is "D&M! Cola". The font and the spelling makes it read like "DEMI Cola".
The pictures all around the walls and desks of Carter's home are the actual childhood photos of the actress Selena Gomez herself from a young age
Demi Lovato and Selena Gomez were real life best friends but they have grown apart.
In the scene where Rosalinda is reading a book to a group of kids you can see Demi Lovato's real life younger half-sister Madison De La Garza sitting at a table.