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The gap between 15-year-old students who are excelling and those who are failing has widened despite the Government's increased focus on the "tail" of educational achievement.
A range of problems from poverty to curriculum changes were cited for New Zealand's poor performance in OECD league tables, in which it fell in all three core subjects since the last assessment in 2009.
It was New Zealand's first big drop in the rankings - from 7th to 13th in reading, 13th to 22nd equal in maths and 7th to 18th in science.
Education experts said yesterday that New Zealand should not get too distracted by its position on the league tables in the Programme for International Study Assessment (Pisa) report.
University of Auckland lecturer and Pisa expert Fiona Ell said: "People get very hung up on the ranking ... because it's like a Top of the Pops top 10 thing. I don't think they should be ignored ... but knee-jerk reactions to rankings are really dangerous in education systems."
New Zealand students traditionally did better in reading than other areas, and the 2012 test comprised mostly of mathematics. The rise of East Asian countries whose learning was similar to the Pisa testing was also a significant contributor to New Zealand's fall.
Academics said that underlying the data that formed the league tables were some worrying statistics, in particular the rise in underachievers - the gap between those students who were doing well and those who were not had widened in the last three years.
This was despite the Government's determination to eliminate the tail of the education system.
Dr Ell said: "In some ways it's surprising in that there's been a lot of focus on priority learners and there's been a lot of work done in this space."
The drop has taken place under the National-led Government, but it said the results were part of a gradual, decade-long decline and blamed a range of factors which it inherited.
Education Minister Hekia Parata said these factors included a new curriculum which was still bedding in, high rates of exemptions in schools, poor data on student achievement and under-investment in teaching practice.
In an urgent debate in Parliament which was prompted by the Pisa results, Ms Parata reaffirmed her commitment to helping "five out of five students" - a reference to the belief that one in five students was failing in education.
Asked whether more money should be channelled into lower decile schools to lift up the low achievers, she said that a $110 million investment in poorer schools had not improved results. She said better targeting of investment was needed.
Not only had the achievement gap widened, but the number of people who were considered high achievers had also decreased in the Pisa study.
The Prime Minister's Science Adviser, Sir Peter Gluckman, said: "What's worrying is that there seems to have been a decline in the people represented in the top end of the scale and an increase in the number of people at the bottom end of the scale."
New Zealand was one of just two countries in which socio-economic status had a strong connection to a student's performance. Some countries' education systems made up for social disadvantage, but this was not the case in New Zealand.
While academics, MPs and unions said it was easy to identify causes of New Zealand's drop in ranking, they said it was more difficult to provide solutions.
Sir Peter said there was no quick fix to the problems with New Zealand's educational achievement, part of which was a cultural problem.
Educational achievement was set early in life, and it was hugely important to address primary teachers' low confidence in science and maths teaching.