The boy was arrested Wednesday afternoon at Our Redeemer Lutheran School. A teacher found the gun in the student's book bag.
North Platte officer John Stadler says it was a scary moment, "but, fortunately, no one was in any danger."
The school was locked down after the arrest, but the administration let parents pick up their children.
The boy's name has not been released. He faces charges of carrying a concealed weapon, uncontrollable juvenile, terroristic threats and use of a firearm to commit a felony.
Stadler says the boy was described as a "wannabe" gang member.