The student was arrested Friday in a hallway of Papillion-La Vista High School, and the gun was found in his backpack.
Principal Jerry Kalina said in a message to parents that the student recorded the video in a student bathroom and posted it to Snapchat. Another student who saw the video reported it to school officials, and police were immediately called.
Kalina says there was no threat made by the student to other students or staff.
A district spokeswoman says possession of a gun on school grounds results in an automatic expulsion.