The incident, captured in a now-viral video, unfolded as school was being dismissed Thursday.
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That's when a group of people pulled up to the campus in a car and got out, according to a message from Pomona Unified School District Superintendent Darren Knowles to parents and staff.
Footage shows some sort of altercation happening when a person dressed in black pulls out a gun and cocks it before pointing it at the fleeing students.
""My son is nervous, obviously, he felt very afraid," said the mother a student. "He was leaving when the fight happened. All he said to me there was someone was armed."
David Montanez, whose daughter is also a student, said she was in the gym when the gun was allegedly waved at her.
"She was terrified, my nieces, my family," he said.
ABC7 spoke to one student, with their parents' permission, who was not at school when the incident occurred.
"It was a bit surprising but it's not completely surprising because incidents have happened -- luckily, outside of the school," said Theo Guerrero, a sophomore.
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It's unclear who the people in the car were, but the superintendent says they're not students. The video, along with witnesses, helped confirm that.
Parents told Eyewitness News that rumors are circulating that current or suspended Diamond Ranch students were in the car.
"At the end of the day, these are gunmen," said a student's mother, who declined to give her name. "That's terrorist -- the way they ... they behavior, how they came in. They came in masks. It's just sad. So they knew what they were doing and they knew where to go. They knew where to find whoever they were going to find."
The superintendent says one student was assaulted in the process. So far, no arrests have been made.
"We are working with the local authorities to provide information to help them apprehend the suspects. Los Angeles County Sheriff's Department will be on campus tomorrow along with additional Pomona Unified School District Security to support the safety of our students. We will continue to keep our parents, students and staff updated as we learn additional information," Knowles said in the message.