North Aurora Police Lt. Scott Buziecki said the grenade couldn't have exploded because the powder had been removed, the Kane County Chronicle of St. Charles, Ill. reported.
Buziecki said he did not know why the boy had taken the device to school.
Officers from the Kane County Bomb Unit were called to the scene Tuesday after staff members were told the boy had a bomb in his backpack.
A school district spokesman said the building wasn't evacuated since the bomb squad determined the grenade was inoperable.
"They evacuated the item rather than the students," said spokesman Mike Chapin.