Thousands of students stayed away from school on Wednesday as members of the Post Primary Teachers Association resumed industrial action over their stalled pay claim.
The PPTA this week rejected the Ministry of Education's latest pay offer and has planned seven more days of action, affecting 100,000 students.
Wednesday's action saw union members refuse to teach Year 9 students, while on Thursday they will refuse to teach Year 11 pupils.
Schools offered to supervise students who did turn up on Wednesday, but it appears most followed their advice to stay away.
Principals say they are trying to minimise disruption to students, but they say they want an end to the dispute.
The PPTA is planning further strikes over the next five weeks, beginning with a refusal to teach Year 10 students next Wednesday.
It has planned three more days affecting Year 9 students this term, and three days affecting Year 10 students.
The union has warned members their fight could continue into next year if the dispute is not resolved.
It has exempted members in earthquake-hit Canterbury from the strike action because of a damaging aftershock in the region on Tuesday.
Money's tight, ministry tells teachers
Education Minister Anne Tolley says striking teachers must understand the ministry is operating under tight economic conditions and there is not a lot more money for pay increases.
Ms Tolley told Morning Report on Wednesday the PPTA wants a 4% pay increase, which teachers already received last year when many others were losing their jobs.
The minister says the union should be at the negotiating table talking with the ministry about the issues that are important to them in order to get a resolution.
She says any complaints from teachers about class sizes should be taken up with a school boards of trustees, not the ministry.
Striking teachers won't lose pay
Secondary teachers will not lose any pay for their strike action this week, though wages would normally be docked for such action.
Ministry of Education workforce group manager Fiona McTavish says the decision not to dock their pay was made in the interests of good faith and to encourage the PPTA back to the bargaining table.
She says the ministry wants to talk with the union about its priorities - class size, pay, and call-back days during school holidays.