A teacher at P.S. Dupont Middle School in Wilmington, Del. alerted authorities after finding a gun inside a 13 year old's backpack.
The handgun was unloaded, police say.
School officials searched the teen's backpack after students told a teacher that they saw the teen walking to school with the gun.
Police say the student did not make any threats towards students or school staff.
Investigators say the teen brought the gun to show off to friends. He allegedly told police he thought it was a BB gun.
Police are checking how the student got the gun.
The teen is being held by police, pending an arraignment hearing.