A government adviser thinks allowing senior students in Auckland and Waikato back to school next week poses a significant risk of increasing case numbers.
Year 11 to 13 students in level three areas can go back to class after the long weekend, the government announced yesterday.
Principals and the teachers' union, the PPTA, have criticised the move, with the latter saying the government is showing "reckless disregard".
Dr Dion O'Neale from Te Pūnaha Matatini, which provides advice on the Covid-19 response, told Morning Report even a small number of cases at schools could lead to widespread transmission.
He said infectious students would act as a link, spreading the virus between families that would otherwise not come into contact with each other.
There was also the risk that if people in the students' households were going to work there was the chance for further transmission.
"So a relatively small increase in the number of infections in the schools by connecting up these different parts of the country or the community that would otherwise not be connected up ... [leads to] an increase in the number of infections overall but much greater than just the number in schools."
The government's plans for better ventilation in schools and the wearing of masks will help the situation but schools and families have been given only two working days to prepare to make the environment safe.
The measures being taken fell far short of a "gold standard plan for returning to school", he said.
"They are at the absolute lower end of what we would expect to be put in place to reduce transmission."
Dr O'Neale would like to see portable filtration systems in each classroom which cost about $1000 each.
He also suggested the use of rapid antigen testing where results are available in 15 minutes.
"It lets you break those chains of transmission before they develop."
An Auckland school principal said there are mixed emotions about returning to class next week, and plenty of questions still to be answered.
Manurewa High School principal Pete Jones told First Up there is a lot to work through before then, including how to manage students and staff who can't return.
He said the last time students were able to return to school, it took weeks for attendance to stabilise and he expects many won't return due to Covid-19 concerns, or because they now have jobs.