King will star in the 15th episode of the season as a student called Brianna, E! Online reports.
When Jess (Zooey Deschanel) discovers that Brianna has been picking on someone, she tries to get involved, only to find herself the new subject of Brianna's bullying.
King recently starred in the movie Crazy, Stupid, Love and has also signed up for a role in NBC's upcoming new comedy Bent.
Meanwhile, Harris will star in New Girl as the vice-principal of Jess's school and is expected to make her debut in next week's episode.
Harris has previously had guest spots in shows such as Cougar Town, Modern Family and Party Down and also appeared in the movie The Hangover.
New Girl returns with a new episode in the US on Tuesday on Fox. The show airs on Fridays on Channel 4 in the UK.
> Dermot Mulroney wins role on Zooey Deschanel's New Girl