The Post Primary Teachers' Association says senior students and unqualified substitutes are taking classes because of a shortage of teachers at some secondary schools.
The union is seeking a pay rise of 4% for about 95% of secondary teachers, which it hopes will address recruitment and retention problems.
PPTA president Kate Gainsford says some principals in provincial areas are desperate for teachers.
She says a school that used senior students to take classes was covering for teachers on sick leave, because there are not enough relief teachers available in some areas.
Ms Gainsford says some high school principals have been holding Expo style stalls in England to try to attract teachers to New Zealand.
Shortage may reduce subject choice
The Secondary Principals' Council says a critical shortage of teachers is forcing some schools to stop teaching some subjects.
The principals' council chair, Julia Davidson, says a pay increase for teachers would help schools to avoid canceling classes.
She says options include classes going onto the correspondence school role or using videoconferencing to access teaching in another school.
But Ms Davidson says sometimes schools only option is to decide not to teach a subject which is a very hard decision to make.
She says teachers taking higher paying jobs overseas and a lack of graduates are two of the reasons for the teacher shortage.
Twice as much as nurses, others got
The 4% pay rise being sought by the PPTA is twice as large as that achieved by other public sector workers.
The health sector recently announced a 2% deal covering nurses and other workers and police officers got 2% last year.
In addition to the pay rise, the teacher union wants laptops for all secondary teachers and classes kept to a maximum of 30 students.
The total value of the claim is more than $100 million per year.