Mason heard gunshots while he was outside with classmates, waiting for school to start. Then students were running, and a teacher was lying still on a playground basketball court, hit by gunfire.
The 12-year-old wanted to help him, but a vice principal told him to run. So Mason moved toward the building -- and that's when he saw Jose Reyes, a friend and a Sparks Middle School classmate, with a gun about 10 to 20 feet away.
"I (said), 'Please don't shoot me, please don't shoot me,'" Mason told CNN in an exclusive interview Thursday from a hospital where he was being treated. "I looked at him. I saw (the gun), and he braced it and shot me in the stomach."
Authorities say Mason was the last of three people that Reyes, 12, shot with a 9 mm handgun Monday morning outside the school. Mason and another wounded student -- the first to be shot -- survived.
Math teacher Mike Landsberry, the second to be hit, died. Investigators say Landsberry probably saved lives by walking toward the shooter -- giving others time to flee -- on the basketball court after the first student was shot in the shoulder.
Reyes fatally shot himself, police said. Sgt. Greta Woyciehowsky of the Sparks Police Department and Adam Mayberry, a spokesman for the city of Sparks, confirmed Reyes was the shooter.
Mason, shot in the abdomen, was able to walk at the hospital Thursday. His mother, Jenifer Davis, said the bullet missed vital parts and exited behind his right hip.
"I'm ... very lucky I still have him here," Davis said.
Now Mason is left wondering what caused his friend to hurt him and a classmate, and kill a teacher that Mason deeply admired.
Mason said the incident began when he heard a few gunshots -- he thought they were in the distance -- as children gathered outside the school after 7 a.m., waiting for the day to begin.
At first confused about why schoolmates were running, he saw Landsberry lying wounded on the basketball court. A friend called 911. Mason and other students wanted to get Landsberry into the school, he said.
"We tried, and one of my vice principals ... said, 'Get in the building, get in the school fast.' And that's when I ran to my backpack, and I (eventually) got shot."
Reyes -- a friend Mason had met at the start of the school year -- said nothing as he shot him, Mason recalled.
When Mason was shot, he said, he felt a sharp pain in the abdomen and ran from the shooter. He soon saw a security guard, who told him to lie down and put pressure on his wound, Mason said.
Authorities haven't said why they believe Reyes opened fire. Earlier this week, a 13-year-old Sparks Middle School student told CNN that many people speculated that bullying could have played a role
Reyes, according to the student, "was yelling stuff like, 'Why are you laughing at me? Why are you doing this to me?'" during the shooting.
Mason said he wasn't aware of any bullying.
"I don't think he was being bullied at all ... because if he was being bullied and I saw it, I would have stuck up for him. I was one of his friends," Mason said.
He said the shooter was nice, but sometimes started arguments with people he disagreed with.
"Anything you're talking about, (if) he doesn't agree with it, he would start arguing ... with that person about what the person said. I (saw) him doing it a lot at school," Mason said.
Mason said he doesn't believe Reyes sought him out or the other wounded student during the shooting.
"We never bullied him. We never (did) anything to" him, he said.
"After he shot me, I thought that he wasn't my real friend," he added.