Auckland secondary schools are already planning catch-up classes and holiday lessons for students who have fallen behind because of the city's lockdown.
The city's Secondary Principals' Association said some senior students had missed so much they would need to come back next year instead of leaving school at the end of this year.
By the end of this week, Auckland students would have been through two lockdowns that turned more than eight weeks of classroom time into home-based learning - two-and-a-half-weeks more than students in the rest of the country.
Principals told RNZ some students had coped better than others with learning from home.
Year 12 student, Kaia Jamieson, told RNZ she had not found it too hard to keep up with learning during the lockdown, but she knew others who had struggled.
"My school's really flexible in the ways in which we can learn and in taking our own time to make sure we get things to the right standard. Some of my friends, especially my friends in other schools, find it really stressful and feel really disadvantaged by lockdown."
Jamieson said her school's emphasis on self-directed learning had helped a lot.
"Most of my friends who have that mindset seem to be doing quite well, but there are a couple of people I know who are prioritising other things over learning at the moment and who are falling behind because of it."
Auckland Secondary Principals' Association president Steve Hargreaves said many of the city's schools were worried about how much learning their students had missed.
"Right across the board students have lost close to a term now, and this is Year 9 to 13, so I'm considering the whole student body here. If they've lost that much time we can't just carry on in 2021 thinking it's business as usual."
He said schools were thinking about asking some students who were planning to leave at the end of the year to return next year to complete credits.
"Those students who had planned to leave may not have the requirements they wanted to have before they left school so they need to possibly come back and be with us at school for a short time, only a month or two to attain those credits."
Hargreaves said a only small number of school-leavers at his school, 4 or 5 per cent, might need to come back next year.
However, he expected all students returning next year would need catch-up classes of some sort.
"I think we're talking about the whole cohort there. Let's say we missed some crucial topics of maths, we haven't covered a section of algebra, we haven't covered a section of statistics in Year 9, then we need to do all of that work for the whole group before they can progress with Year 10 mathematics," he said.
"You can't just roll into Year 10 if you've lost a quarter of Year 9."
Hargreaves said universities and polytechnics had already indicated they would make concessions for this year's school-leavers.
The principal of Sir Edmund Hillary Collegiate Senior School, Kiri Turketo, said it had been running extra after-school classes since the first lock-down ended in May, and those would continue.
"As soon as school finishes at 3pm we give them a half-hour break and then we go from 3.30 to 5pm. So we'll be doing a lot of that, possibly the weekends because we've got some amazing staff that will give up a day in their weekends to come in for our students.
"We'll be doing a lot of catch up."
Turketo said the classes were active lessons rather than supervised homework, and students were keen to attend.
"It's got to a point now where they expect that, which is good, because they know and we know that they aren't where they're supposed to be for a lot of reasons.
"The social and economic ramifications of the lockdown have made our vulnerable community even more vulnerable."
Turketo said the school wanted to ensure learning was not a barrier to its students' life choices and it would work with them as long as necessary to help them get qualifications.
The principal of Ormiston Senior College, Diana Patience, said the school had a flexible timetable and most students who were behind should be able to catch up during time normally scheduled for independent learning.
However, she said the school was ready to do more for those who needed it.
"There are a number of teachers already volunteering that they would be likely to be running tutorials in the holidays, so the goodwill amongst our staff is fantastic."
Patience said the school was also considering using time usually reserved for teacher professional development to run extra classes in practical subjects as hospitality and music.
"We've even talked about bringing students back for a Year 14 if they need it for a term next year."
However, she expected only a few of the school's students would need extra help to catch up.