More asbestos dust has been found scattered throughout Kaipara College since it was closed a week ago.
More than 100 tests have since been done and principal John Grant said positive readings had come back from the school hall, the administration building and the music and drama areas.
He said the rest of the school had been found to be safe, with no asbestos detected in the air.
Mr Grant said the hall roof would be cleaned and sealed on Sunday, in the hope that school would resume as normal for the college's 650 students on Monday.
In the meantime, senior students preparing for exams have been receiving tuition in local facilities such as sports clubs.