Auckland Regional Public Health Service was working to identify any close contacts - any students and staff members who may have been closer than 2m to the student for more than 15 minutes - of the four students.
Parents have been asked to keep their children at home until contacted by the ARPHS. Staff were also advised to stay home from school.
Principal Vaughan van Rensburg said public health officials informed the school of the case at 2pm on Wednesday.
"The community has been really supportive and we're looking forward to seeing the kids back at school," he said.
Chapel Downs will undergo a deep clean and will reopen next Monday, pending advice from public health officials.
Parents were urged to remain vigilant of Covid-19 symptoms in their children, including a new or worsening cough, a sore throat, runny nose or fever.
"Finally, if you know the identity of the student or family with Covid-19, please do not disclose their names or details to anyone else, and discourage your children from speculating or identifying them on social media," parents were told.
"This can lead to online bullying and abuse."
Just one new case of Covid-19 was reported on Wednesday, a recent returnee to New Zealand who tested positive for the virus while in managed isolation.
The woman in her 30s arrived from Dubai on September 9 and was linked to the three border cases announced on Tuesday.
But one whānau is grieving the loss of a second loved one, who died of Covid-19 within two weeks of his brother.
Nigel Te Hiko died on Wednesday in Waikato Hospital, where he has spent several weeks in ICU.
The 54-year-old is the youngest person in Aotearoa to die from Covid-19.
It is believed he contracted the virus from his brother Alan, who died in Auckland Hospital on September 4.
Bloomfield said Te Hiko's family had a message for New Zealanders: "The man's whānau has asked us to tell the country that coronavirus is so real and to be vigilant and cautious.
"They have issued a plea to all New Zealanders: If you are sick and have symptoms stay home and seek advice about getting a test."
Bloomfield said the man's death highlighted the seriousness of Covid-19 and said his thoughts were with Te Hiko's family.
"Ngāti Raukawa has lost clearly a rangatira... they mourn the loss of their loved one.
"I can't imagine how devastating this is for this whānau," Bloomfield said.