The 11-year-old is a student at West Kearns Elementary School, 4800 S. 4620 West, and brought the gun to school in his backpack, Horsley said.
The weapon was a .22-caliber handgun. The boy also had ammunition, although it was not immediately clear whether the gun was loaded or if the bullets were the right size for the gun, Horsley said.
It was not clear how the boy had obtained the gun or whether it was from his family's home.
Some witnesses have said they saw the boy brandish the gun on the playground and point it at another child's head. Police have not yet been able to confirm those accounts, Horsley said, noting that it is sometimes difficult to sort out the facts when all the witnesses are children.
Two of the boy's classmates told their teacher about the gun at about 3 p.m., about 45 minutes before the end of the school day.
The teacher immediately secured the boy and took him to the principal's office. It was the principal who retrieved the boy's backpack from his classroom and contacted Granite police, Horsley said.
No one was injured. The school was not placed on lockdown.
"In this case it was clear that the danger had been mitigated and lockdown was not necessary," Horsley said. "If there was any indication that student safety was being threatened we would have implemented a lockdown."
Granite district schools are equipped with security cameras, doors that require electronic keys and other security measures that aren't often discussed publicly, but Horsley said the greatest security system in any school is the "eyes and ears of our students."
"We do talk to our students about it and encourage them to take personal responsibility for keeping their school safe," he said. "We are very grateful that a student made a trusted adult aware of the situation."
District police continued to interview the boy Monday evening, and his parents were cooperating with school administrators and police, Horsley said.
The boy was expected to be taken to juvenile detention Monday night, he said.
Under Utah law it is illegal to bring a weapon to school.
"At a minimum the police will be charging him with possession of a weapon on school property," Horsley said.
Additionally the boy could face brandishing and aggravated assault charges if police find evidence that he had the gun in the open or threatened another student, Horsley said.
A faculty meeting to discuss the incident was planned for Tuesday and the district planned to make grief and guidance counselors available to help staff effectively discuss the issue of guns in school. A key part of the message will be to remind students of the importance of sharing information with teachers and staff, Horsley said.