Teachers are angry the New Zealand Qualifications Authority blocked some students from logging into online exams this morning, including the level one English NCEA exam.
The authority said it was prepared for about 20,000 logins to three online exams, but noticed the system slowed when more than 18,000 students joined the system.
It stopped further logins and instructed candidates to complete a paper version of the exam.
"NZQA apologises for the disruption caused for some students and will continue to work with its technology vendor as they scale up the capacity of the system," it said.
The online exams offered this morning were level one English, level two Lea Faka-Tonga, and level three Media Studies, however, only those trying to sit their English exam were affected.
Candidates for the other two exams were able to log in using a URL provided by exam centre managers.
Level one English is one of the biggest exams and one teacher told RNZ this morning's failure was unacceptable.
The teacher said some students were sitting new level one English standards and no paper copies of that exam were provided, meaning teachers had to print them out.
Association of Teachers of English vice-president Pip Tinning said a lot of teachers were angry about the disruption particularly due to a similar problem occurring last week with online NCEA literacy and numeracy tests.
The authority had stopped students logging into online numeracy tests for 45 minutes, due to a high number of logins.
After that problem, the authority said it aimed to improve forecasting of when it would need additional capacity for online exams.
"It seems a bit problematic that a week later we've got the same kinds of issues cropping up," Tinning said.
She said the disruption caused mayhem and some students did not cope well.
"Because it's level 1 for a lot of our kids is one of the first really big exams that they've sat and just the anxiety and the breakdowns actually for some, awful, absolutely awful and absolutely unacceptable," she said.
"It's a lot of pressure for a 15, 16-year-old kid to overcome."
Tinning said teachers had told her that the alternative URLs that NZQA provided for other exams, including the level 1 exam featuring new versions of the English standards, did not work for all students.
She said at one school log-ins to the exam stopped just one minute after it was due to start and teachers had to figure out which students were logged in already and which needed to have paper versions of the exam.