The Visitation Elementary School, located at 851 S. York St., was placed on soft lockdown during a search of the school about 9:30 a.m. Wednesday, police said.
No weapons were found.
An investigation determined the student brought the ammo to show other students.
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Police said another student told a parent, who then contacted school administrators.
No further information about the student involved would be released, police said.
Elmhurst police planned to maintain an additional presence during school arrival and dismissal.