New Zealand families paid $12.5m towards the cost of NCEA exams last year (file photo).

Thousands of high school students passed their exams last year, but did not get the certificate because they failed to pay.

Education advocates have called for changes to the "unfair" NCEA (National Certificate of Educational Achievement) exam fees, which led to about 5500 students' qualifications being withheld in 2016.

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One in six students received financial assistance for NCEA fees last year. Advocates say the fees threaten the right to a free education, but lawyers warn challenging current legislation will be difficult.

Schools collect an annual per-student NCEA entry fee of $76.70. It contributes to the costs of setting, holding and marking exams on behalf of the New Zealand Qualifications Authority (NZQA). The fee has not increased, except to account for GST changes in 2010.

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NZQA has collected more than $137 million in examination fees since NCEA was established in 2005.

About 158,000 students' families paid $12.59m in NCEA fees last year.

Nearly 27,000 students received financial assistance for their NCEA fees, worth $1.58m, in 2016. That was about 3500 more than in 2014, but fewer than in 2015 (28,171 students). More than $19.1m in financial assistance has been granted since NCEA began.

Financial assistance, which must be applied for before September 1 each year, is available to children of beneficiaries, community services card holders and families where the NCEA fees exceed $200. This reduces the fee to $20 for individual students or $30 for a family.

Because fees are payable after exams, some students do not apply for assistance, miss the December 1 payment deadline, or never pay at all. About 60 per cent of those who did not pay last year did not pass NCEA, NZQA said.

Minister of Education Nikki Kaye said NCEA fees should not be a barrier to students achieving their full potential, hence the financial assistance.

New Zealand Principals' Federation president Whetu Cormick questioned the system's fairness.

"Why is there a $76 fee? Why can't families have that paid for them?

"We're about all children having an equitable education and one in six [students seeking assistance] is a significant group. It's a big number of families that just reflects the dire situation many are in – they just can't afford it."

The practice of withholding qualifications over non-payment of fees was "a critical issue for the Government to consider", Cormick said.

Post Primary Teachers' Association (PPTA) president Jack Boyle said families made a "huge" contribution to education that state funding did not cover.

He said the Government's right to charge parents NCEA fees appeared at odds with its legal obligation to provide free primary and secondary school education.

"That's what the tension is, is that overriding principle of free education suggests that you need to have a change of the law."

Chen Palmer partner Nick Russell said "valid political or philosophical" arguments about barriers to free education could not trump legal authorisation to charge NCEA fees.

"If that financial assistance is available then it would show that there's not an impediment. The most you could argue is that it [NZQA] should be reviewing its fees, if one in six can't pay it."

Taylor Shaw senior associate James Pullar said problems would arise if fees became too expensive.

"Certainly there would be a point at which that's something Government would have to look at. If you've got one in six families asking for financial assistance that's something for Government to consider.

Secondary Principals' Council chairman and Darfield High School principal James Morris said he preferred schools approached families for NCEA fees rather than "external agencies that might come across a little heavy handed".

"We remind them as gently as we can to pay. Its clear for some families that the process of completing the paperwork is a barrier in itself . . . probably for some others it's just another secondary school fee."

HOW IT WORKS

- Students or their families must pay $76.70 annually to sit NCEA exams, usually held in November.

- The fee is due by December 1.

- Families can apply for financial assistance with exam fees by September 1. It makes it $20 for a student and $30 for a family.

- Those who do not pay can view their results online, but are not awarded the qualification.

- Last year, more than 13,000 students did not pay their exam fees. Of those, about 60 per cent did not pass.

- NCEA level 3 is required for university entrance.