Teachers at Te Aho o te Kura Pounamu, formerly known as the Correspondence School, say they are at breaking point because of problems with their new computer system.
The Post Primary Teachers Association (PPTA) compares the situation to the problems that dogged the Novopay school payroll system.
The $12 million student management system was put in place in the country's biggest school in March 2012.
The school has been criticised over its handling of the new system, in particular by two independent reviews last year which its board refused to accept.
The PPTA has surveyed its 200 members at the school, and nearly all of the 159 who responded said the system is slowing their work and causing stress.
The union says the schools' senior managers have been saying the system is working fine, but outside intervention may be needed.
The Educational Institute has members among the schools' primary and early childhood teachers and says they are finding the school's system difficult to work with.
The Correspondence School says its managers have not yet had a chance to discuss the survey with the union. But it says it shows there is a wide range of ability among staff using the system, with some able to do tasks in a relatively short time and others taking longer.
The school says it is committed to making the system more user-friendly.