Rolling year levels of secondary students are being rostered home for the rest of the term as teachers begin another round of industrial action.
Union members are back at the negotiating table with Ministry of Education officials this week, after rejecting the government's latest pay offer.
The ministry said teachers were offered three pay rises totalling between 11 and 18 percent by December next year, along with lump-sum payments of up to $5,210 for union members.
Under the latest industrial action, teachers are refusing to teach two year levels per day over the next three weeks and will not attend meetings or respond to emails out of school hours, along with other work bans.
PPTA Te Wehengarua acting president Chris Abercrombie said teachers were fighting for a pay rise that matched inflation.
"No-one's happy about this, everyone is frustrated. Teachers don't want to be doing this, they'd rather be in the classroom," he said.
Abercrombie said current and prospective secondary teachers were turning their back on the profession because of poor pay and conditions.
"There are schools right now where you can't take certain classes because they don't have teachers. There were schools already rostering students home before our industrial action because they didn't have enough teachers," he said.
"It's a great career, but unfortunately when you're making decisions about paying your bills, teaching for a lot of young people isn't stacking up at the moment."
Abercrombie said he knew of at least two schools that did not have any specialist science teachers for their junior classes because of worsening staff shortages.
"You're not getting that specialist knowledge, that specialist understanding, and no parent would accept that as okay," he said.
"We want to make sure there are subject specialist teachers in front of every student in Aotearoa New Zealand, that's what they deserve and that's what we're fighting for."
Ministry of Education employment relations manager Mark Williamson said three days of facilitated bargaining would resume on Wednesday.
"We are disappointed that the PPTA has chosen to take further action. We don't believe that industrial action, which impacts student's learning, should continue while bargaining is underway," he said in a statement.
"Strikes and stopping our young people from learning and participating in school life will not help reach a settlement and only delays teachers from receiving the benefit of the considerable investment that our offer makes in remuneration and conditions."
Drop in teacher education programme enrolments
Universities have reported a big drop in teacher education programme enrolments, with about 600 fewer primary students than last year and about 65 fewer secondary students.
The figures followed forecasts of a secondary teacher shortage but a potential over-supply of primary teachers.
Primary teachers voted to accept their latest pay offer last week, following a lengthy campaign that included the biggest education strike in the country's history.
However a union survey of principals by NZEI Te Riu Roa found 47 percent of new principals - those in their first or second year in the role - planned to quit the job in the next five years.
None of the 629 respondents said they felt well supported, while 79 percent said the demands of the job were "difficult" or "impossible" to manage with the staffing and resourcing available.
'We can't afford for people to leave the sector'
NZEI president Mark Potter said principals found their work hugely rewarding but desperately needed more support.
"Even new leaders in the profession are facing burn-out and ultimately, it's the children who will suffer if we can't attract people to the role and retain them," he said.
Wainuiomata's Arakura School principal Seletute Mila, who was in her third year in the role, said the demands of the job were unsustainable.
"I'm doubly disadvantaged being in a smaller school and having a school that has really high and complex needs. Those are the key reasons why for us, it's like being in a pressure cooker every single day and not seeing a way out," she said.
"We need more teachers in schools and if we had that, I would be able to do my job really well, but as it is a lot of things end up back on my plate because there's just no-one else to do it."
Mila said schools were increasingly dealing with complex societal challenges affecting children, including poverty, inequality and the rising cost of living, but she loved her community too much to give in.
"We can't afford to lose principals, we can't afford for people to leave the sector. We need good principals to stay and lead our schools. We need a government and a ministry that actually values the work that we do and resources it properly," she said.