Test results indicate last year's lockdowns damaged children's achievement in writing, but had little effect on their reading and maths.
A Ministry of Education report said by the end of last year, writing scores for children in Years 5 - 7 were lower than in 2019 by the equivalent of more than five weeks of learning.
But for teenagers in Year 9 the gap was nearly 16 weeks and for those in Year 10 it was nearly 20 weeks - even though students lost only five to eight weeks of classroom time last year.
"Progress in writing consistently reduced more in 2020 than for other learning areas for most groups of students. These reductions in progress - while not insurmountable - are more likely to have longer-term implications, and potentially represent the equivalent of more weeks of learning than can be explained by the period of distance learning during 2020," the report said.
"However, we are less certain about the writing estimates, because they are based on fewer students and inherently contain more variability. Even with writing, most differences in 2020 are within the range of regular year-to-year variation."
The study said children's progress in reading and maths was little different to 2019 and where it was worse the dips were similar to fluctuations in previous years, meaning it was not clear if they were due to the lockdowns.
Education Ministry deputy secretary evidence, data and knowledge Alex Brunt said overall the results were similar to previous years.
"End of year learning progress in reading and maths across the system was essentially unchanged for most student groups in 2020. Where we could detect statistically-significant decreases, such as in writing, the changes were very small and well within a range of what we could expect learners to pick up the following the year," he said.
He said the results were national averages, and some schools and communities might have fared worse.
Brunt said other countries were also tracking the impact of lockdowns on their school pupils and New Zealand had done better than most.
"We stack up incredibly well," he said.
Secondary Principals' Association president Vaughan Couillault said teachers would be working hard to address the poor writing results for students who were in Years 9 - 10 last year.
"It represents something that we've got to take action on fairly swiftly. I don't think it's completely disastrous yet because you are talking about students at the lower levels of school," he said.
He said the report underlined the difficulty of teaching writing to students who were learning from home.
"This piece of evidence is proving that writing, which was challenging in the first place, is even more challenging when you're not there to cajole, to encourage, to lever off that teacher-student relationship," he said.
Auckland Primary Principals' Association president Stephen Lethbridge said a five-week hit on achievement in writing for children in Years 5 - 7 was not as bad as teachers were expecting.
"It's less than would be expected given that we did have a significant lockdown and Auckland had the series of lockdowns. That's not too significant," he said.
He said the results were a pat on the back for teachers' work during the pandemic.
"It was a significant issue for schools last year. There was a lot of hard work that was done by a lot of teachers across the country and especially in Auckland with repeated lockdowns. So I think they'll look back on it and think that this report gives them a big thumbs-up for what they did."
The ministry's report said there was no evidence the lockdowns had worsened gaps in performance between rich and poor schools.
It said Auckland schools' results were not much different to those of other schools, even though they lost more class time to lockdowns.