Laura Valdavia has been taking care of Sarah since her mother died of breast cancer in June 2009.
Sarah's father died of prostate cancer in 2001.
Valdavia says Sarah has been depressed since her mother death.
She may have taken a large quantity of Tylenol pills before running away.
Police believe she may still be in the Fullerton area.
She is black, 5-foot-5, 120 pounds, with black hair and brown eyes. She was last seen about 4:40 p.m. Thursday, wearing a green scarf, t-shirt and blue jeans.
She's a student at Troy High School in Fullerton.
Anyone with information about Hightower is asked to call Detective Jose Flores at (714) 738-6754.