Low-decile schools fear the loss of hundreds of students to high-decile schools next year.
Some schools are increasing their out-of-zone enrolments for 2021 to make up for the drop in foreign student enrolments.
They are doing it to avoid job losses, but it could cause roll decline and job cuts at other schools.
The situation prompted the Education Ministry to ask 11 Auckland schools with high foreign enrolments about their intentions for out-of-zone enrolments.
Auckland Secondary Principals' Association president Steve Hargreaves said some schools had decided to increase their out-of-zone enrolments next year in order to reduce the impact of the loss of foreign students.
They were most likely to do that with senior year levels who would be in their school for only a few years while the international student market recovered.
"We're talking tens of students. Schools are really careful not to take huge numbers of out of zone students because they need the capacity because maybe when the border opens up in 12 to 18 months we need to reopen the market for international students," he said.
Hargreaves said it was possible the greater intake of out-of-zone students would harm other schools, but the impact could be offset by the growing population of teenagers in the city.
"The other factor in the background is we all know we've got a growing cohort of secondary-age students coming through so hopefully we're splitting up a bigger pie and in the end nobody has to suffer too badly."
Principals were talking about the issue because enrolments and ballots for out-of-zone places were happening now.
Papatoetoe College principal Vaughan Couillault said the situation was causing anxiety and nervousness among principals, but nobody yet knew what the impact would be.
RNZ understands some schools are planning to take 30-40 more out-of-zone students.
Couillault said 30-40 students did not sound like a lot, but the impact would be significant if several schools were taking that number of enrolments from one other school.
Post Primary Teachers' Association president Jack Boyle said the problem was likely to be most acute in Auckland, but it would happen throughout the country.
"We've got international students across the country. I'm down here in Nelson at the moment and certainly a lot of principals and schools are looking at the impact of losing large numbers of international students. They've already lost large numbers and next year is looking pretty grim. So there are other locations right across the motu that will be in that same situation."
The Education Ministry's deputy secretary sector enablement and support, Katrina Casey, said ministry staff had talked to 11 secondary schools with high numbers of international students "to discuss the impact, if any, that the likely reduction of international students may have on their ballot process for 2021 out of zone places".
"It is important to note that under Section 519(7) of the Act, school teaching space does not include spaces established for international students. Therefore the spaces that a board of trustees provides for international students can't be used for domestic out of zone enrolments," she said.
Casey said the schools were planning out-of-zone enrolments in line with usual annual movements.