Papatoetoe High School principal Vaughan Couillault says the Year 9 student confirmed as one of three new Covid-19 cases was in school for one or two days while infected.
Couillault told Mike Hosking on Newstalk ZB that the reaction to the community was one of "anxiety and a little bit of worry".
"The infectious period she was at school for a very short period of time...we are essentially talking about one or two days," he said.
Couillault revealed 1411 students, 152 staff and about 50 close contacts and their families have been asked to isolate and get tested.
Casual contacts, such as students who were not in the same class as the infected girl, have also been asked to get tested and isolate but the families they live with are not required to at this stage.
Others, such as visitors who were at the school last week are also asked to get a Covid-19 test. The other two cases are the student's mother and father.
Health officials have confirmed the girl was infectious while she was at school earlier in the week and her classmates and teachers are now being asked to get tested and self-isolate until February 24.
Relatives of those classmates and teachers are also being asked to get tested and stay at home until receiving a negative result.
The school will be closed today and tomorrow.
The infected student stayed at home on Thursday and Friday after her mother, an employee at airline catering provider LSG Sky Chefs, became unwell.
Couillault said yesterday that the student did not attend "any assemblies or anything like that" on Wednesday.
"So in terms of identification, we have probably the smallest numbers of teachers and students that we could have for a Year 9 student."
He said five teachers and 28 students are considered close contacts of the infected student and all other students and staff at the school are "casual" contacts.
Couillault has notified all staff and parents by text and email.
An Auckland Regional Public Health spokesperson said letters have been sent to all staff members and to families with students with advice on what to do.
The letters advised staff and parents of students whether the student or staff member was a "close contact" or a "casual contact".
Auckland Regional Public Health Service defines "close contact" as someone who has been in the same class or room as the infected student.
"Casual contact" means the person is at a lower risk of developing the disease because they have not been in the same class or room as the student.
Casual contacts are being asked to get tested and remain at home until they get a negative test result while close contacts are asked to get tested and stay home until February 24.
People at the school have been told to look out for Covid-19 symptoms, such as new or worsening cough, fever, sore throat, temporary loss of smell or taste, difficulty breathing
and runny nose.
A mobile testing centre will be set up at the school on Monday and a list of testing sites is available on the Auckland Regional Public Health Service website.
The girl and her parents have been moved into quarantine after testing positive on Saturday night.
The father is a self-employed tradesman.