The lockdown at Tamahere Model Country School, on the outskirts of Hamilton, has been lifted.
Police were called to the school after senior students saw a stranger behind the school grounds wearing a balaclava and carrying what appeared to be a shotgun, an update from the school to parents said.
The lockdown was lifted after police inspected the school grounds and the area where the stranger was spotted and did not find anything.
Parents who turned up at the school reported seeing the Armed Offenders Squad and police dogs at the school.
In an earlier email to parents just after 1.30pm, the school advised that children had all been accounted for and were safely locked inside.