CBS Chicago reports the boy was cited as a juvenile delinquent Tuesday morning after bringing the device into Jewel Middle School, although it turned out to be a dud.
The Kane County Bomb Squad didn't know that upon arriving at the school. The unit carried the hand grenade outside until it was a safe distance away from the school building.
According to CBS Chicago, officials at the school were shocked to learn the boy had a hand grenade in his book bag. School officials credited students with informing their teachers.
"They did the right thing," said district spokesman Mike Chapin.
Chapin said parents of the students were notified of the incident by school officials.
The district has had other incidents of look-alike weapons brought to school, although parents and students are warned about it at the start of the year.
The boy was taken to the North Aurora Police station, and will be referred to Kane County Juvenile Court Services.