Students at a Newcastle school have been allowed to return to class, after they were evacuated when an 11-year-old girl brought in a hand grenade for "show and tell".

The NSW Police Bomb and Rescue Unit left Hunter Christian School, Mayfield, on Wednesday afternoon, declaring the area safe and allowing students back inside.

A NSW Police spokesman said Australian Defence Force personnel had taken the device with them to examine it.

Police said the grenade resembled a World War II "pineapple" and believed it was inactive.