Teachers say they are "gobsmacked" by revelations of widely varying pass rates for different subjects in national exams.
The average pass rates in the National Certificate of Educational Achievement (NCEA) over the years 2012 to 2016 range from 69 per cent for home and life sciences up to 93 per cent for languages.
Almost half of all language students (43 per cent) achieved "excellence" - almost five times the rate for home and life sciences (9 per cent).
The next-highest rates of excellence were in music and dance (both 34 per cent).
Maths Teachers Association president Gillian Frankcom-Burgess expressed alarm at the comparatively low excellence rates in maths (13 per cent) and statistics and probability (11 per cent).
"I'm gobsmacked, I have to say, because I didn't know that languages have this higher rate of excellence, I've never seen this before," she said.
"And music - a third of musicians are getting excellence. That's unbelievable!"
The figures have been calculated from NZ Qualifications Authority (NZQA) data by the NZ Initiative think-tank in a report proposing a new "weighted relative performance index" to measure students' NCEA achievements, taking account of the different grade patterns in each subject.
The report cites achievement rates for broad fields of study and for selected "sub-fields", some of which include multiple individual subjects. For example, "home and life sciences" comprises food technology and textile technology.
Home Economics and Technology Teachers' Association president Sarah Wirth, who teaches at Samuel Marsden Collegiate in Wellington, said the differential pass rates were unfair to home and life sciences.
"It's not valued, really, and yet it's a very important subject when you look at the skills that come out of those subjects," she said.
The report looks only at achievement standards, which can earn "merit" and ":excellence" grades, but Wirth said food and textile students traditionally sought mainly unit standards and the few achievement standards in those subjects were based on writing about projects rather than practical work.
"You should be able to achieve if you are practical, and you should be able to achieve if you can reflect and evaluate it," she said.
"It's just not about the practical any more. I think that sometimes that has gone too far and we are not looking at the skill set of what the kids make."
The lowest pass rate by broad subject field in 2016 (71 per cent) was in business, which comprises accounting, business studies and business administration.
Commerce and Economics Teachers Association president Jude Maurice said the differential pass rates suggested that NZQA based its grades on a "bell curve", awarding higher grades to more students in "hard" subjects such as languages than in business.
"Every subject should be given their opportunities," she said.
"We have all these interests and career pathways, and students need to feel as though they can excel in those subjects or else they are not going to take them."
However Association of Language Teachers vice-president Dr Martin East said NZQA's "profiles of expected performance" for each subject were based on historical pass rates.
"It may be that we have more able students who are choosing to take languages at NCEA level. That, of course, will increase the likelihood of getting higher grades," he said.
Ryan Timoko-Benjamin of the Dance Subject Association said the high excellence rates in dance and other arts subjects reflected the fact that external exams in those subjects were "very language-rich".
"They are not just performing, they are actually analysing works of art, as well, I'd say, at a higher level," he said.
NZQA referred questions to the Ministry of Education. A ministry deputy secretary, Ellen MacGregor-Reid, said the subject differences partly reflected the numbers of students taking each subject, ranging from virtually all students taking English to small numbers taking languages or music.
"Smaller specialist subjects such as music, dance and languages may see better student performance because of student choice," she said.
"Students, supported by parents and teachers, usually focus on specialist subjects they consider they are good at, alongside core subjects like English and mathematics [which we know most learners continue with into senior secondary].
"Student performance in these specialist subjects can also be driven by learning outside of school. Many students work on their artistic skills, in particular, outside of school or through extracurricular activities, meaning they may make progress more quickly."
Average NCEA achievement rates by selected subject subfield (2012-16)
Languages 93% (excellence 43%)
Dance 89% (excellence 34%)
Drama 87% (excellence 26%)
Visual arts 86% (excellence 26%)
Religious studies 85% (excellence 23%)
Te Matauranga o Aotearoa 84% (excellence 19%)
Music 83% (excellence 34%)
Economic Theory & Practice 83% (excellence 19%)
Accounting 82% (excellence 27%)
Mathematics 82% (excellence 13%)
Social Science Studies 82% (excellence 29%)
Reo Māori 81% (excellence 21%)
Science 80% (excellence 19%)
Statistics & Probability 80% (excellence 11%)
Health & Physical Education 78% (excellence 15%)
English 78% (excellence 14%)
Technology 77% (excellence 19%)
Business Operation & Development 73% (excellence 13%)
Design 72% (excellence 17%)
Home & Life Sciences 69% (excellence 9%)
Source: NZ Initiative