The son of Lisa Marie Miguel, 44, brought the WWII simulated hand grenade to the Johnson Academy in Stratford, Connecticut.
The student told the teacher he brought in the grenade because they were studying World War II.
The students at Johnson Academy were moved from their classrooms to the auditorium and Stratford police along with the Connecticut State Police were called to the scene, the WFSB-TV reported.
"The bomb technician reported the simulated hand grenade could combust and cause damage to property and persons within a 15 to 20 foot radius," police said in a statement yesterday.
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After the item was removed from the school, Miguel talked with police and confirmed that she had given the grenade to her son for the school lesson.
Miguel told police that her deceased father gave the WWII simulated hand grenade to her as gift from his "military days." She said that she thought the grenade was a "dud."
Miguel was charged with risk of injury to a minor, reckless endangerment and possession of an explosive device. She was released with a promise to appear at Bridgeport Superior Court on June 16.