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.