Members of the secondary teachers' union are refusing to teach year 10 students on Tuesday as their collective agreement talks remain stalled.
The latest strike by the PPTA will affect about 60,000 students.
One of the sticking points in talks between the union and the Ministry of Education is class size.
The ministry says secondary schools should try to keep average class sizes to 26.
But teachers and principals say despite that classes can get as large as 40.
The PPTA wants a maximum of 30 pupils, with lower numbers for labs and workshops, and compensation payable when the limits are exceeded.
The ministry says wage bargaining is not the place to set education policy and has offered a working group to look at research about class size.
But the union says large classes raise workload and health and safety issues.
It says the collective agreement is the only place where meaningful limits on class size can be made.