The boy was expected to be charged in juvenile court Tuesday, Horsley said.
"This kid made a mistake, and he knows it," Horsley said. "He feels bad about it, and his parents are cooperating with the investigation. He will not be coming back to this school."
No one was injured.
Two other Utah schools were dealing with rumors of gun possession by students that turned out to be false, underscoring fears spread by the Connecticut shooting.
Separately, Utah's attorney general-elect, John Swallow, said he planned to make school safety a high priority and that fortifying schools might be one solution.
"When we had the issue with the airliners, for example, we strengthened the cockpit doors so that terrorists on the plane couldn't get through to the pilot," Swallow told The Associated Press.
Granite School District officials said they have a high level of security compared to other Utah schools. The district employs its own police force with 16 armed officers on patrol, plus school resource officers who are off-duty police officers.
Associated Press writer Brady McCombs contributed to this report.