City police said the boy left one spent shell casing on a school bus and two more on the floor in a boy's bathroom at Harvard Junior High School, the Chicago Tribune reported.
"The youth reportedly brought three .30-caliber spent shell casings to school to show friends," police said in a release. "The youth subsequently realized the consequences of his actions after losing one on the school bus, so he discarded the other two shell casings at school."
The boy, who had taken the casings from a collection at his home, was processed by police and released to a parent, the newspaper said. School officials and police wouldn't discuss what punishment the boy would face, the Tribune said.
The discovery led officials to lock down all district schools, with students in their classrooms while all lockers and other areas were searched.
Harvard Community School District 50 officials said the incident was unrelated to a Monday incident in which custodians discovered a shell casing in the cafeteria at Crosby Elementary School, also in Harvard, a community of about 9,500 people in northeastern Illinois' McHenry County.
"In light of the tragic and recent events that have occurred throughout the country, the Harvard Police Department would like to remind everyone to be vigilant of their surroundings," Harvard Police Chief Dan Kazey-Garey said.
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