The Northcote student was infectious when at school on August 16 and 17, the email said.
All staff and students at Northcote are considered close contacts and have been told to self-isolate at home for 14 days.
The North Shore school has about 1200 students.
It's unclear whether the student is one of the already confirmed 21 cases.
Health officials have also added a bus route from Glenfield Mall to Northcote College to their ever-growing locations of interest list. The route (95B or 95C) travelled between the locations from 8.27am to 8.40am on August 17.
The Lynfield College pupil was also infectious while at school on August 16 and 17. Staff and parents of students were notified of the positive case earlier on Thursday evening.
Staff and students there are also being told to self-isolate at home for 14 days.
Again, it is unclear if the student is part of the cases publicly reported by Ministry of Health officials to date.
Lynfield College is located in Mt Roskill and has a roll of about 1800 students.
This comes after students and staff at Avondale College were told they would have to quarantine for two weeks after a teacher was identified as having Covid-19 earlier in the week.
Meanwhile, a student at the NZ School of Tourism has also been confirmed as having Covid-19. It is unclear whether the student is one of the already announced cases or additional.
The school was unaware of when the student had become infected, the email said. The student had not been on campus since August 10.
On August 9 and 10, they had attended a class at the school's Queen St site from 9am-12pm.
STORY CONTINUES AFTER LIVE BLOG
STORY CONTINUES
Today, it was revealed Auckland's community positive cases were a close genomic match with a recent returnee from Sydney on a managed red zone flight on August 7.
That person returned a positive result on August 9 and was transferred from Crowne Plaza to Jet Park on that same day.
More than 100 locations have now been identified as potential Covid exposure sites across Auckland, with the list now stretching as far south as Manukau.
The latest count, as of the Ministry of Health's list updated at 5.10pm, shows 124 visits to 101 sites.
However, New Zealand's Delta outbreak may now be "more contained" than first feared, a virologist says, after overnight genome sequencing secured a critical piece of the puzzle.
But experts still warn that the scale of the outbreak, which has so far grown to 20 cases, could still number around 100 infections.
Today, it was revealed Auckland's community positive cases were a close genomic match with a recent returnee from Sydney on a managed red zone flight on August 7.
That person returned a positive result on August 9 and was transferred from Crowne Plaza to Jet Park on that same day.
As officials could now work on the assumption the current cases came from the traveller, a search for the missing links in the infection chain was underway.
"There's a genomic match to a case in MIQ, which strongly suggests this may be - or at least close to being - an index case of this cluster," ESR and Otago University virologist Dr Jemma Geoghegan said.
