Kaipara College in Helensville has been closed temporarily after tests found asbestos dust.
The dust was discovered in the school hall and the administration block.
Principal John Grant said further tests were being carried out to work out how much of a problem it posed.
"It's a swab sample that has detected the asbestos at the moment. We are running air monitoring but we won't know the results of that until Monday."
Mr Grant said it appeared the asbestos had come from the school hall's old fibre-like roof that dated back to the 1960s.
Parents were being asked to regularly check the school's website for updates.
Kaipara College has 560 students.