When they spotted another student with a gun, they reported it to administrators.
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"Sanger High is a campus that addresses matters rather quickly," says Sanger Unified School District Superintendent Eduardo Martinez.
The Sanger Police officer assigned to the campus found the student and detained him -- the officer found a loaded ghost gun on the student's waistband.
A ghost gun is essentially a homemade weapon.
People order different parts and assemble the weapon. It has no serial number and is untraceable to law enforcement.
Sanger police searched the student's home and found other gun parts and tools, which leads detectives to believe he may have been assembling firearms at home.
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Tuesday night, the district is advising parents to speak with their children about what happened as a lesson to be alert and report unsafe situations.
"To our parents, we appreciate the continued support in letting their kids know when they're aware of a situation that is not safe, then we need to notify administrators, notify teachers, campus safety officers," Martinez said. "We're all here to provide a safe environment for all students and staff at all times."
That student was arrested and is at the Fresno County Juvenile Justice Center -- facing multiple felony charges.
Police say they're still investigating why he brought that gun to campus.