Police found a teenager with a loaded gun near campus after responding to a report of a disturbance at Newark Memorial High School.
The find caused officials to put the school on lockdown around 11 a.m.
A SWAT team was also called to campus.
Police said the teen was among a group of students who scattered after they arrived to investigate the disturbance call. Some of the students ran on to campus, according to police.
Police spokesman Matthew Breen said the SWAT team searched for the people who caused the disruption.
Police said they thought one of the them remained unaccounted for and that delayed the call that the scene could be cleared.
A woman who answered the phone at the school, located at 39375 Cedar Blvd., emphasized that all students were safe.
A police command center was set up at Newpark Mall. That is also the location parents were told to go if they want to pick up their children.