Forrest, a senior at Menlo-Atherton High School, is one of several high school students in the production set in a clinic for emotionally disturbed teens.
Last summer he was cast in the workshop for the new play, which took place at the Royal Bath Theatre in Bath, England.
"Riot" by Irish playwright Ursula Rani Sarma is the newest commission from A.C.T.'s partnership with the Royal Bath Theatre.
Forrest, the son of Terri and Joseph Tiffany, has been performing with A.C.T. since he was 9, appearing for several seasons in "A Christmas Carol."
He attended Phillips Brooks School and Hillview Middle School before entering Menlo-Atherton. He has been accepted in the fine arts acting program at Emerson College in Boston this fall.