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Schools in some of New Zealand's poorest areas have achieved the biggest gains in student achievement over the past six years.

Kaikohe's decile-one Northland College, where more than 95 per cent of the 305 students are Maori, has lifted its pass rate for NCEA level one by Year 11 from just 11 per cent in 2004 to 65 per cent last year - above the averages for all the four poorest deciles of schools.

De La Salle College in Mangere East, another school in the poorest tenth of schools with a roll that is 86 per cent Pasifika, raised its pass rate from 28 per cent to 63 per cent.

The biggest percentage increase, leaving aside schools that did not exist five years ago, was from 10 per cent to 100 per cent at Murupara's tiny Rangitahi College - but there last year's 100 per cent pass rate represented just seven students.

In general, average pass rates increased faster at poorer schools, partly because of what Waikato University's Professor Russell Bishop called "a ceiling effect" in the richer areas.

"You get to a point where it's difficult to do much better," he said.

Northland College principal John Tapene said schools like his had also put huge efforts into lifting achievement.

"A lot of it is not just with the students but also with the families, getting them to understand what we are doing and why," he said.

Northland College has also worked heavily on professional development for teachers.

It is one of 50 schools in Professor Bishop's Te Kotahitanga programme for Maori students, and is part of a programme for Maori teachers. About 60 per cent of its teachers are Maori.

It is working with other Northland schools to lift the number of students leaving school with at least NCEA level two from 68 per cent at present to 75 per cent, and uses an active apprenticeship scheme as "bait" to keep students at school.

De La Salle College has already reached 99 per cent of school-leavers achieving at least NCEA level two. Acting principal Brian Evans said his college was also getting parents involved.

"We get literally hundreds of parents turning up to parent-teacher evenings. The numbers have increased every year," he said.

Pass rates have dropped since 2004 at relatively few schools, with most big falls in very small schools where a handful of students each year can swing the figures.

The statistics have been affected by a growing number of schools now offering alternatives to NCEA.

Auckland Grammar School headmaster John Morris said the schools offering British-based Cambridge exams had increased from about 20 in 2004 to 58 this year.

"We were very disappointed with what was happening with assessment in 2001, when we brought Cambridge here. There was far too much internal assessment," he said.

"There is going to be even more internal assessment mandated by NCEA. We are very likely next year not to offer NCEA level one at all."

He said the top 65 to 70 per cent of Grammar's students were streamed into Cambridge exams, leaving only the bottom 30 to 35 per cent sitting NCEA.

Other schools offer all students a choice of either Cambridge or NCEA, with the numbers opting for Cambridge ranging up to 75 per cent at Macleans College and about 80 per cent at King's College.

Another 11 schools on the NCEA list, including Auckland private schools Diocesan and St Cuthbert's, have opted for another overseas system, International Baccalaureate.