The Post Primary Teachers' Association is calling for an independent inquiry into problems dogging the Correspondence School's new computer system.
The union says the school appears to have mis-managed introducing the system, which delayed enrolment for new students and prevented teachers from assigning or marking work.
The $12 million system was introduced in March this year and the Correspondence School says it is now largely in place.
But it still has problems - data has not yet been transferred from the old system and it does not automatically provide NCEA results to the Qualifications Authority.
The PPTA says the process has not been well managed and it wants an inquiry.
The Correspondence School says it had already commissioned independent reviews of the project and the last of those - into the implementation of the system - is nearly complete.
It has not committed to publishing the findings, but says core functions of the new system are now in place.
Parents of those enrolled with the Correspondence School are worried the problems will harm their children's NCEA results.
The school has 24,000 students a year and parents say some did not receive school work for up to 10 weeks.
The Parents and Supervisors Association is worried the delays and problems could impair children's performance in exams later this year.