The child arrived with the grenade in his hand at the school in the southern town of Kristianstad on Tuesday.
A teacher who saw the grenade contacted police.
Police officers were able to determine it was a “dangerous” grenade the boy had picked up this summer at the Rinkaby firing range, located outside Kristianstad.
They blocked off the area and the national bomb squad neutralised the ammunition at the scene several hours later.
“We don’t know how bad the damages would have been” if it had exploded, a police spokesman told AFP.