The Post Primary Teachers' Association says the school year is off to a shambolic start because the Education Ministry seriously underestimated the number of returning students.
Union president Kate Gainsford says more than 400 teachers were made redundant at the end of last year despite schools' predictions of higher rolls.
Yet with the recession and youth unemployment, Ms Gainsford says, it was obvious that more students than usual would stay on. Now classrooms are overcrowded, she says, and schools are scrambling to find more teachers.
The head of one of the country's largest secondary schools, Rangitoto College principal David Hodge, says with youth unemployment at 18.4%, calculations should have been adjusted.
Mr Hodge says his school has 50 more Year 13 students than the ministry predicted. He says trying to hire extra teachers after the beginning of term is difficult, and quality can be an issue.
Not an exact science, says ministry man
The ministry's resourcing manager, John Clark, says the system used to predict the numbers of senior students is not an exact science. It relies on roll figures for previous years, he says, and all schools have the chance to respond to provisional figures.
The president of the Secondary Principals' Association, Peter Gall, has some sympathy with the ministry. Because more students who can't get jobs are staying on at school, he says, roll predictions are harder to make.
The ministry will always be conservative with predictions, Mr Gall says, and schools do have licence to employ staff based on the number of students they expect to have.