Deputies responded to the scene, the 7000 block of Mullen Road, at about 7:15 a.m. after receiving a report of a child struck by a vehicle that did not stop.
Lt. Greg Elwin of the Thurston County Sheriff's Office said investigators quickly found evidence that led them to believe the boy was struck by a 17-year-old girl who attends nearby Timberline High School.
Elwin said deputies believe the 15-year-old boy was walking west toward school when someone across the road offered him a ride. The sheriff's office said he darted out in front of the girl. They say it does not appear that she was texting or talking on her cellphone.
Deputies said once she got to school, the girl immediately told staff she'd hit somebody in the road.
News spread quickly around the school.
"That's pretty messed up," said student Zach Morrison. "She shouldn't have left. She should have stayed right there."
"I could see how she'd be scared and keep going," added Whitney Benfield. "But in my opinion, I would stop and try to help or call the cops or something other than just leave him there."
Students say the crash site is a dangerous stretch of road with no lights or crosswalks. The sheriff's office said because the teen darted out in front the girl, it's likely she wouldn't have faced any charges. But by leaving the scene everything changed. She faces the potential for felony charges to be filed for leaving the scene of an accident.
"If you're involved in car crash any time you need to stop -- particularly if you think you hit somebody," Elwin said. "You need to stop and render aid right away."
Medics treated the boy at the scene. He was taken first to Providence Hospital, where he was prepared for airlift to Harborview Medical Center in Seattle.
Harborview officials say the boy came out of surgery Monday evening and was listed in serious condition.