Schools say classrooms poorly designed for good airflow are hampering efforts to "add another layer of protection" for students against Covid-19.
NIWA suggests opening windows and doors would help with ventilation needed to dilute airborne droplets of the virus, but school leaders say it's not that simple - especially come winter.
Principal of Tauranga Boys' College Robert Mangan said ventilation in classrooms was a "complex issue".
In his view, classrooms were not that well-designed to meet the ventilation requirements that were encouraged or desirable to reduce Covid.
Mangan said the school would be "pro-active" once Covid was in the community.
"Once there was the need, we would be recommending to all of our teachers that all windows were open both sides [to] get airflow [and] doors remain open."
He said the constraints of physical design and planning for ventilation flow through most classrooms was "not great".
"In some cases, you'll have upper windows open on both sides but a relatively limited flow of air through the classrooms.
"We'll do the best we can, based on the facilities we currently have."
Mangan said keeping windows and doors open during summer would be "practical steps" the school could take under the constraints of the design.
Arataki School principal Shelley Blakey said doors and windows would be left open during break times on cold days to ensure there was ventilation.
Blakey said Ministry of Education advice about making sure there was "clear airflow through classrooms" had been helpful.
"That's something that we encourage the teachers to ensure that happens."
Rotorua Girls' High School principal Sarah Davis said the school was single-level, which was "good news" for ventilation and meant it was relatively easy to just open windows.
"At the moment, the weather suits for us to be able to follow good practice but we might be a wee bit more challenged if we're still struggling with Delta in our community into the winter months next year.
"We'll certainly have to try to work out how we can keep our classroom spaces warm plus have the ventilation that would support the safe practice of teaching."
Davis hoped that by winter the ministry would have released more guidelines about keeping classrooms safe when doors and windows could not be left open.
"I'm hoping we get information that's going to assist schools like us... how we can manage that those classrooms can be as safe as possible," Davis said.
"I'm certain they'll be able to walk alongside us to put us in a place where we're following best practice."
Post Primary Teachers Association president Melanie Webber said some schools were having to work "quite hard" to get airflow due to the way some classroom blocks were constructed - such as in the shape of the letter H.
"Depending on which way that's facing it can be very difficult to get the airflow with that. Part of the building is blocked so you don't always get great ventilation."
In some schools, windows had been painted shut and did not open, she said.
"Teachers are doing the best they can."
In areas under Alert Level 3, Webber said it was good that there were limited numbers of students back because they could be moved to classrooms with better ventilation.
"But as we get whole schools back that's going to be much more difficult to manage."
NIWA air quality scientist Ian Longley agreed that winter was "much more challenging" and difficult.
"Obviously what we don't want is cold, wet, damp kids - that will have a lot of serious health implications in itself, let alone making them more vulnerable to Covid.
"We're trying to figure out what a practical and reasonable solution would be. We'll be doing some sort of testing and trials over the summer holidays, basically to trial some ideas."
Longley said ventilation carried away the virus before someone else had the chance to inhale it.
If there was an infected person in a room, the air would stay there and eventually everyone in the room would be breathing in that virus, he said.
"But if you've got doors and windows open, air just carries it into the outside world. It's diluted, it's gone before everyone else has the chance to inhale it."
Longley recommended that schools should be open to review how well classrooms were ventilated.
The same advice applies to all indoor spaces, including businesses.
Longley said businesses should be able to open in Auckland if customers did not need to go inside.
"That's because as long as the customers are outside, that slight movement of the wind is enough to stop them transmitting the virus. But once you go inside, you're sharing that air and the risk rises."
New Zealand Institute of Education Te Riu Roa president Liam Rutherford said ventilation was "a major issue" and it was working with overseas unions to see what was working there.
"At some point, all schools and early childhood centres will need to review their current health and safety policies."
Ministry of Education infrastructure and digital leader Scott Evans said its advice to schools was that fresh air was "the best way" to ventilate a space and encouraged opening windows, doors and vents.
Schools were asked to reach out to their caretakers to repair windows that did not open and to contact the Ministry if they had any concerns around ventilation, he said.
"We are actively supporting a small number of schools who have contacted us with ventilation concerns and will continue working with those schools to find the right solution for them."
Evans said the Ministry understood keeping windows open would not always be possible, especially in winter, and it was working with air quality panels for more advice about how to best manage indoor air quality in their classrooms during the cooler months.
All new school buildings were required to meet the Building Code requirements in place when they were built, which includes standards on ventilation, he said.
"We also have design standards that our architects, designers and engineers use to ensure our new and refurbished buildings meet the Ministry requirements for internal learning environments. This includes indoor air quality and ventilation."
The ministry was investigating the use of purification and filtration, and when these would be beneficial for supporting air quality, he said.
"All schools and kura must be able to meet public health requirements when staff and students are on site.
"Along with requirements for testing, vaccination, good hygiene and physical distancing, indoor learning areas also need to be well ventilated."