Chaos erupted shortly after the opening bell when the child at the Salvador Castro Middle School opened fire and shot two of her classmates.
Among the wounded was a 15-year-old boy who, despite being shot in the temple, was in stable condition, and expected to fully recover.
A 15-year-old girl was shot in the wrist, and three other people suffered minor injuries.
No motive was given for the 9 a.m. local time shooting, but at least one parent described the school as a haven for bullies and gangs.
Even so, one report cited a student who said the girl thought the weapon was a toy gun.
Television footage showed the suspected shooter wearing a sweatshirt with her hands behind her back.
Cops arrested her at the school, officials said, and recovered the weapon used.
Two students — an 11-year-old boy a 12-year-old girl — and a 30-year-old woman were injured but not shot during the incident, officials said.
“We will attend to the needs of these students, the witnesses, very carefully, with the understanding that this is very traumatic,” said LA School Police Chief Steve Zipperman at the scene.
The shooting sparked panic in the Westlake District of Los Angeles, and came just a week after two high school students were killed by a classmate in western Kentucky.
The gunplay and ensuing lockdown sent worried parents scrambling for answers about their children.
Jocelyn Lopez told local news channel KTLA her 13-year-old sister was sitting near the two students and “just heard something pop inside the classroom.”
“It was really close to her,” Lopez told the channel. “Not even the school is safe … it’s just bad.”
Rosario Hernandez, 41, told the Los Angeles Times she got a phone call from her 16-year-son, Jimmy Romero, telling her a shooting had occurred at his brother’s school.
Hernandez left work and sped to the school. She texted Johnny and asked him if he was OK.
When he finally responded, he told her they were still on lockdown and said the shooting had happened inside a seventh-grade classroom.
“She shot a girl and a boy,” he wrote.
“OMG,” Hernandez responded. “But why?”
Classes later resumed, and counseling was offered for anyone distressed by the shooting.
It wasn’t clear if the school had gone through one of the district’s required random checks, in which officials use wands to root out any weapons at schools.
Doctors said the boy shot in the head was initially awake and alert and answered questions
Aaron Strumwasser, a doctor at the L.A. County-USC Medical Center, where the wounded were being treated, said the boy escaped death by just inches.
“It could have been devastating,” Strumwasser told the Daily News. “In this particular circumstance, had the bullet traversed from low to high, it would have been a devastating, probably fatal injury for the kid. Because it went from high to low, it probably saved his life.
“This kid lucked out,” he said. “It could have damaged vital vessels that supply the blood flow to the brain, and it could have damaged the brain itself, significantly, but he lucked out. Nothing fatal or catastrophic was identified. And I expect him to make a full recovery. Hopefully he’ll be back on his feet and going to school again, under safer conditions.”