The MPMS librarians recommend use of the DoInk! green screen app for Apple device owners. There are two versions: a $2.99 version and a $5.99 version. The $2.99 version is what's used here at school.
Below are some tutorials recorded by Melanie Farrell, one of the WCPSS Digital Learning Coordinators.
This video provides a general overview of how to properly record a green screen video.
This video covers how to import and edit video in DoInk!
This video shows how to create green screen puppet shows.