Auckland's Marist College has three new confirmed cases of coronavirus.
A staff member at the school was already confirmed to have the virus on Sunday.
In a letter to parents, principal of the Catholic school for girls, Raechelle Taulu said all staff and students at the college were considered close contacts.
"This means that families need to assume that their daughter has been in contact with a teacher or student that has Covid-19. However, this does not mean that your daughter will get the virus, yet we have a duty of care to inform you," the email said.
Marist would begin its school holidays from tomorrow until April 14, Taulu said.
"If your daughter is well, I encourage her to continue accessing her learning online. Please be mindful that teachers are also using this time to concentrate on their health, but when and where possible, will update their online learning platforms."
A letter from the Ministry of Health to parents said it was "likely your child has been near one or more of the cases".
"While you will be worried about your child, the chances of he or she developing the illness are small."
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Children and young people were very unlikely to catch the virus, it said, and if infected they would probably have only mild illness.
However all students would still need to self-isolate from today until April 5.
During the upcoming lockdown, most New Zealanders would still be able to visit essential services such as supermarkets - however people in self-isolation faced further restrictions and should not leave their homes at all, the letter said.
Other household members were not considered close contacts of confirmed cases and could continue with normal level 4 restrictions from tomorrow.
The school announced on Sunday it would close for 72 hours after a staff member tested positive for coronavirus. The school would be cleaned and contact tracing would begin.
The person and their whole household was in self-isolation, an earlier letter to parents said.
"They are being supported at home and will only return to school after they have completed their isolation period and are well," Taulu said.
"We have been planning for this in terms of being prepared for students to work remotely from home - like many schools.
"Marist is an amazing community so I know that at the end of the day we will all look out for each other."