St Dominic's Catholic College in Auckland will be closed for at least three days after a student tested positive for Covid-19.
The Auckland Regional Public Health Service (ARPHS) said it and the Ministry of Education were now working with the College to identify close contacts of the case.
Staff and parents were told on Tuesday afternoon.
Any close contacts will be getting further information soon on self-isolating and testing.
The student was last at school on Friday but left early after starting to feel unwell.
Health officials said the family did all the right things arranging for the student to get tested over the weekend and notifying them on Tuesday morning.
The student and their family are in isolation.
In a statement, ARPHS said: "The whole school community has been asked to remain vigilant for the signs of Covid-19 and to call their doctor or Healthline for advice, and get tested, if they do become unwell.
"ARPHS is also aware of a number of students from Liston College who attended classes at St Dominic's, but considers these students to be casual contacts only.
"A close contact of the case who attends Henderson North School is self-isolating and has been tested, but remains well and was very unlikely to have been infectious when last at school."
It said while the news would be of concern to families and staff, the risk was low for the vast majority of the school communities.
- If you have symptoms of the coronavirus, call the NZ Covid-19 Healthline on 0800 358 5453 (+64 9 358 5453 for international SIMs) or call your GP - don't show up at a medical centre