It was a show-and-tell gone wrong.

A child in England unknowingly brought a live World War II-era explosive device to his school to show off to his classmates — and the device ultimately had to be detonated by a military bomb squad, according to a local report.

The vintage shell — which was originally believed to be inert — was determined to possibly still be active after it was brought in for show-and-tell in a class full of 9- and 10-year-olds in the county of Devon, about 200 miles west of London, Plymouth Live reported.

The incident unfolded last month at the Kingsacre Primary School when the student brought in his great-grandfather’s medals along with a historic empty shell case.

The items “were displayed to the class but at no time were they handed around to pupils,” the school’s head teacher Claire Cole told the news outlet.

“The next day the artifacts were collected” by the student’s grandfather, Cole said.

A concerned parent at the school told Plymouth Live that the shell was left in the school overnight until the child’s grandfather realized the device might still be active.

“The grandfather collected it from the school, put it in the boot of his car and drove it to Barnstaple police station,” the parent said, adding, “He went into the police station and explained what he had in the back of his car. His car was surrounded and the military were called.”

Authorities then took the device to another location, where it was detonated as a precaution, officials confirmed.

“My concern is that the school have yet to inform all the parents of this massive breach in health and safety and the safeguarding of our children,” the concerned parent told the news outlet.

Following the incident, Cole said the school “clearly will need to review our procedures for show-and-tell and what is brought into school.”