The wrist wound to the 15-year-old girl was considered minor.
The girl was reportedly shot in the wrist, and the weapon used is now in custody.
Authorities say Samuel Woodward, 20, of Newport Beach killed 19-year-old sophomore Blaze Bernstein and buried his body in a neighborhood park not far from the Bernstein family's home in Lake Forest.
The two victims, one 15-year-old boy and a girl of the same age, were transferred to nearby USC Medical center.
Jordan Valenzuela says he heard a bang and screaming from a next-door classroom at Salvador B. Castro Middle School on Thursday morning.
The district has a policy that requires every middle and high school campus to conduct daily random searches by metal-detector wands at different hours of the school day for students in the sixth grade and up.
Apparently, this shooting was an accidental discharge of just one round from inside a backpack.
We are fully cooperating with the Los Angeles Police Department in its investigation of this matter.
"We will have counselors on site for students and staff (Friday) at Sal Castro", Los Angeles Unified School District spokeswoman Shannon Haber said. A 30-year-old woman suffered a minor injury in the chaos after the shooting, officials said.
Police received a call about the shooting at the school in the Westlake North neighbourhood at 8.55am local time and responded.
Castro has about 365 students in grades 6-8 and nearly all are Hispanic and many from low-income families.
In a separate development Friday, a Southern California man charged in the killing of a University of Pennsylvania student appeared in court to face a murder change. The school was placed on lockdown, which later was lifted.
Benjamin Urbina, a seventh grader in the classroom when the shooting unfolded, told reporters later the girl did not know the firearm she'd brought to school was real.
"We have our school mental health folks who are here to support the needs of the students and we will continue to work with those students", he said.