Four-year-olds will attend some primary schools under wide-ranging reforms set to pass into law, alarming opposition parties and the primary teachers' union.
NZEI president Lynda Stuart said the change is a departure from the "special" New Zealand tradition of starting school once a child has blown out the candles on their 5th birthday cake.
"Our view was the status quo should remain," said Stuart, long-time May Road School principal. "Having those opportunities to socialise and learn in those early childhood settings is really important."
Labour, the Green Party and New Zealand First have also raised concerns, saying kids could get "lost" in a system they're not ready for.
The Education (Update) Amendment bill is down to be read a final time this week and is a key plank of the biggest education reforms since 1989. New Zealand's current system allows children to start on their 5th birthday, and requires consistent attendance only from the age of 6.
For many schools that will remain the case.
However, for school boards that opt to change to the new cohort option, students will start from the beginning of term closest to their 5th birthday - although parents wouldn't have to enrol their child until they turn 6, as is presently the case.
The earliest children could start is up to eight weeks before they turn 5.
Labour's education spokesman Chris Hipkins said there was much in the legislation that Labour supported, but overall it was opposed because of concerns including the cohort entry system and new communities of online learning.
"You should start the cohort after your 5th birthday. They are just too young. New Zealand is already a bit of an outlier - most other countries you would be 6 or 7 before you would start primary school," Hipkins said.
The legislation is also opposed by the Green Party and NZ First. In its minority view on the legislation, NZ First stated almost no submissions were lodged in favour of lowering the earliest school age to 4 years 10 months, and the change appeared to be for the benefit of administrators and funding equations.
Nikki Kaye, who became Education Minister this month after taking over from Hekia Parata, said there was a good reason for allowing some 4-year-olds to enrol.
"If children were required to be aged 5 when starting with a cohort, this would result in school-ready 5-year-olds having to wait longer to start school, and would place increased pressure on capacity in the early childhood education sector.
"It's really important to note that no child will be required to start school before their 5th birthday. In fact, parents won't legally be required to send their children to school until their 6th birthday."
Kaye said schools will be required to consult with their communities before introducing cohort entry. There is evidence that starting school in a group helped children settle and establish strong relationships.
"I don't think it is unreasonable to allow parents the option to let their children start at school a maximum of eight weeks earlier than their 5th birthday."
Peter Reynolds, chief executive of the Early Childhood Council, said he saw no issue with 4-year-olds starting school close to their 5th birthday.
"It is not a nominal age that suddenly prepares you for school. There are some 5-year-olds who are not ready, and they need to wait awhile. There are some younger children who are arguably well and truly ready.
"It's really about the individual child and how ready they are for school. Very much that's going to be influenced by the dialogue and relationship between the parent and early childhood teacher."
Reynolds said the cohort entry change was a good idea for some schools, and would make administration easier.
"But you have to be transparent about it - it has nothing to do with the learning outcomes of children, it's got everything to do with saving some administration time for schools. I can't blame schools for having a look at it."
Reynolds said an issue was a number of ECE centres could struggle if cohort entry at a nearby primary school meant a group of kids left at once. There could also be a perverse incentive for some parents to take their child out of ECE as soon as possible to save on fees.
University of Auckland senior lecturer and early childhood researcher Jean Rockel said she personally felt there was little reason to allow 4-year-olds to start school.
"If you look at countries that have great success with their children at school, such as Finland and Scandinavia, you'll see they start school around 6 years old."
In a pre-Budget announcement last week Prime Minister Bill English outlined new "social investment" spending, including a new programme to support pre-schoolers with oral language needs and literacy difficulties.
The Herald has previously reported on primary schools around the country noticing a decline in the spoken-language abilities of new entrants, with some not able to speak in sentences.
Another controversial aspect of the law change is to allow school-age children to enrol in an accredited online learning provider instead of attending school.
Any registered school, tertiary provider such as a polytechnic or an approved private company or organisation will be able to apply to be a "community of online learning" (Cool).
Each Cool will determine whether students will need to physically attend for all or some of the school day. Such detail would be provided before the Education Minister signs-off accreditation.
The existing correspondence school, Te Aho o Te Kura Pounamu, will become the first accredited online provider.
Letting able kids start early gives parents more options
Changing the age when children start school will let parents make the decision that's best for their child, Michyla Corban says.
The Pokeno mother of two has one son who's brilliant and one who just plods along.
Her older boy, Stevyn, is 4 and three quarters.
He can differentiate between volume and mass, talk about centrifugal force, and will happily explain different sources of renewable energy.
Intellectually he was ready for school aged 4, Corban says, and she would send him now if she could - but he turns 5 in July so he won't start at Harrisville Primary School until the first day of Term 3.
But his 2-and-a-half-year-old brother Michael is not academically inclined.
"At 18 months, Stevyn could count to 20 in two languages. At the same age Michael still called me Dad," Corban says. "I just watched him eat a box of raisins - he ate the cardboard instead of the fruit."
Michael's birthday is in November but he will not start school until the following year, because "his intellectual and social skills are nowhere near where his brother's were at the same time".
She says as long as parents retain a choice in the matter, it's great that some kids will start school earlier.
"I know one of my kids will be ready earlier and I know one isn't going to be. It would be nice to have the option to send the able one early, but at the same time you shouldn't have to push the slower ones."
• From next year school boards will be able to choose whether to change to a new "cohort" entry model.
• For school boards that opt to change to the new cohort option, students will start from the beginning of term closest to their 5th birthday. Parents will have the choice to start their child up to six weeks before their 5th birthday in term one, two and three, and up to eight weeks before their 5th birthday in term four.
• Families won't be legally required to send their child to school until their 6th birthday, as is currently the case.
• How many schools will take up the option is not known but NZEI expects a significant number will, particularly larger primary schools.