Moran Biton, the 18-year-old driver of the sport utility vehicle, was arrested on suspicion of felony hit and run, police said.
A neighbor, Scherrie Payne, heard the impact and called 911.
"I grabbed the phone and heard a couple of kids yelling, 'Oh, no. Oh no," Payne said. "I ran out and I saw the body. I started praying for him. I knew he was gone."
Police said the teenage driver kept going and finally pulled over several blocks up the street.
"She apparently saw police personnel that were on an unrelated call," said Capt. Bill Sutton of the Los Angeles Police Department. "She approached them and said, 'Hey, I think I hit someone.'"
Lynch was described as smart and well liked by his classmates.
"He was always nice to everybody, not just some people," said classmate Josh Anderson. "He was really smart. I always used to ask him questions about math. He was just a great guy."
"I'm horrified," said Charlie Deakins, a friend of Lynch. "I'm just really sad that this happens."
Students on Wednesday will gather for a prayer service in the gym.
The school said grief counselors will be available throughout the day.