Kamo High School, in Whangārei, went into lockdown after a threats were received by the school.
The school went into lockdown about 11am and police and emergency service personnel were stationed around the school.
Police said an 18-year-old boy in Kawakawa was helping them with enquiries.
Staff and students were released from lockdown at 11.37am.
The school told parents the lockdown was a precaution and would update them when it could.
"Your child(ren) are safe and all appropriate measure are in place. We will advise of any updates as they come to hand. Please stay away from the school."
The threats were directed at individuals at the school, police said.
Police are also investigating what they have now confirmed to have been a bomb threat at Te Puke High School this week.
Armed police were called to the Tui St college on Monday morning after the principal said the school had received a malicious email. Te Puke Police officer in charge Sergeant Mark Holmes said inquiries were continuing.