The number of teens leaving school with no qualification is much higher than officially reported at nearly one-in-five of last year's 70,786 leavers.

The figure has been under-reported because since 2004 the Education Ministry has counted students half-way to a level two or three NCEA certificate as if they had a qualification, even if they didn't.

Including those students took the number of school-leavers with no qualification equivalent to at least NCEA level one to 13,165 last year - 19 percent of the total number of leavers and well above the previously reported figure of 10,496 or 15 percent.

Figures provided by the ministry also showed the number of school-leavers with 30 or more credits but no qualifications grew by 150 percent from 1058 in 2019 to 2669 last year, a period in which the total number of school-leavers increased 19 percent.

Adjusting 2019's result to include leavers with 30 or more credits but no qualification took that year's percentage of students with no qualification to 13 percent.

The practice of counting 30 or more credits at levels two and three as equivalent to a lower level certificate in the school-leaver statistics was no secret - it was included in the definitions used for each level of achievement - but it was a surprise to principals spoken to by RNZ.

Papakura High School principal Simon Craggs said he had no idea about the methodology and he did not agree half a level two or three certificate was equivalent to a level one certificate.

"For getting level one, particularly up until recently, you needed to have 80 credits so it's actually a lot hader to get level one than it is to get level two or three currently in terms of the number of credits accumulated," he said.

But Craggs said completed achievement or unit standards carried some value, even if students did not have a full NCEA certificate.

"A prospective employer can look at the things that a student is able to do and that's one of the strengths of the NCEA system," he said.

"Something that we've been advocating strongly for in the new qualification is that there is a list of what students can do and it's more strengths-based rather than they just passed or failed."

Steve McCracken from the Post Primary Teachers' Association (PPTA)'s secondary principals council said counting qualifications in the way the ministry did for its statistics way would make a big difference to schools' results - but they were not allowed to do that and it would not be true.

"I don't think it would be accurate would it, it would be a little bit of falsification of information really," he said.

"It's not right and it's not accurate so it needs to be rectified."

McCracken said schools relied on the ministry's figures to benchmark their own performance so it was important they compared like with like.

He said changing the interpretation of the results made a big difference to the school leaver figures.

"That's a significant increase isn't it in terms of people leaving without any formal qualifications," he said.

The ministry's data and insights general manager Grant Pollard said the practice was historical.

"The decision on that methodology was made at the time to reflect the transition in school quals. So 2002, NCEA level one was phased in as the primary qualification for Year 11 students replacing School Cert," he said.

Asked if the ministry decided at the time that 30 or more credits at levels two or three represented as much learning as a lower-level certificate, Pollard said it was possible to infer that but he could not definitively say why the decision was made.

He said the ministry would consider if it needed to change the methodology given a new qualification would be introduced from 2028-2030.

"We do need to look at it going forward," he said.

"And given the increase in the number of the 30-plus we're going to do some further work to try and understand what might be driving that."