Police said Sade Sweetwyne went to school on Aug. 31 but didn't return and hasn't been seen or heard from since. They said she may be in the Oakland-Berkeley area.
Sade ran away from home once before but on that occasion she returned the following day, police said.
She was last seen wearing a yellow shirt, a white jacket and a cream-colored skirt.
Oakland police said Sade, who is black, is 5 feet tall, weighs 120 pounds and has brown eyes and black hair.
They said anyone who sees Sade or has information about her whereabouts should call them at (510) 238-3641.