The incident happened inside Scott High School Monday morning, the Toledo Blade reported.
Lt. Mark King, a spokesman for Toledo police, said the teen was spotted with the gun outside of his backpack earlier in the day and was later isolated in a hallway where police talked to him through a negotiator.
It was unclear why the boy brought the gun to school and what he planned to do with it.
School officials said the boy faces immediate expulsion.
Parents of other students were allowed to pick up their children Monday at 11:30 a.m. at the school's field house. Classes will resume as normal on Tuesday.