A police bomb squad removed the device and cordoned off the area around the school after the student turned up at a morning class with the grenade, the Telegraph reported.
Police said the girl’s family had received the explosive device from a friend several weeks ago and thought it was a dummy.
The grenade that has been taken for examination by Defence Force munitions experts is believed to have been an inactive device, possibly from the First World War.
The school’s headmaster, Boyd Allen, said that the girl’s teacher brought the grenade to him and he immediately evacuated the 450 students and 60 teachers and called the police.
“I wasn’t sure if it was a dummy and I didn’t want to take that chance,” he said.
Allen, from the Hunter Christian School in Newcastle, said that the girl would not be punished.
“She’s bewildered, embarrassed – I tried to make her aware she’s not in trouble.
“She’s a sweet young lady from a lovely family. She understood it to be a dummy hand grenade that had been deactivated, there was no firing pin, just the body of the grenade. It was heavy, but I assume practice grenades would weigh that much too,” he said.
Police said the grenade had been inspected by Defence Force experts, who took it for testing and would probably destroy it.
“There’s no doubt that it did pose a potential threat and the actions we’ve had today are appropriate in the circumstances,” Police Inspector Gerard Lawson said.
“In a classroom, certainly if it was live and it was a fully functioning device it would cause serious injuries to all the occupants,” he said.