The crime happened Monday morning at Indian Trail High School and Academy.
Police said they were tipped off to a snapchat photo which allegedly showed a 16-year-old boy with a weapon.
The boy was searched at the school and then arrested.
The gun wasn't found in his possession until a later search at the detention facility, police said.
The boy is now facing several charges.
An internal review is being done to determine if any mistakes were made in the initial search for the gun, police said.