The film, written by "Cake" producer Kristin Hahn, is an adaptation of the 2000 novel by Jerry Spinelli, said The Hollywood Reporter.
The book centers on a homeschooled teenage girl who enrolls in an Arizona high school and through her non-conformity alters the ecosystem of the student body, while at the same time capturing the heart of star-struck boy.
But when the student body turns on her, the boy tells her to go against her nature and try to fit in.
King, 16, will play Stargirl, while Plummer, 16, will play the boy who narrates the story.
Plummer is best known for playing lead role of Timmy Sanders on the series "Granite Flats".
