San Bernardino County sheriff's spokeswoman Cindy Bachman said 8-year-old Joshua Robb ran out of the gates of Grandview Elementary School's playground in Arrowhead at about 11 a.m. Monday.
Staff gave chase, but the boy ran into the forest and has not been found, Bachman said.
Law enforcement was on scene within minutes of receiving the call that a boy was missing from the school, Bachman said.
"He's rather quick and this kind of behavior where he runs off is typical," said Bachman, adding that to her knowledge he'd never been lost in the woods before.
Joshua is "severely autistic and does not respond well to loud noises or loud voices, and unfortunately in this situation we have to have the helicopter searching for him so I'm sure that is somewhat disturbing to him but it's absolutely necessary," Bachman said.
The boy is 60 to 75 lbs, with brown hair and dark brown eyes. He was last seen wearing a tan and grey-striped collared shirt, black shorts and grey sandals.
Search and rescue teams from San Diego and Riverside will join the search effort at daybreak, Bachman said.