Our poorest schools are swapping nearly half their pupils a year, as transient families chase work or flee debt.
Some schools say they have taught 7-year-olds who have been through eight schools in their first two years.
Many transient children also have learning difficulties but are often uprooted before schools can bring in extra support.
A decile 1 school will, on average, have twice the student "churn" of a decile 10 school, according to Ministry of Education figures. During the 2013 school year, a typical school in a highly deprived area would have lost and gained the equivalent of nearly half its roll.
A decile 10 school typically has a much more stable roll, with about a quarter coming or going last year. This does not include pupils starting or finishing their schooling.
The transience was even worse in primary schools, hitting children at a time when experts say moving schools is the most harmful.
The figures, released under the Official Information Act, show Russell School, a decile 1 primary in Porirua, had the highest level of pupil turnover in the Wellington region two years ago.
Principal Sose Annandale said a Housing New Zealand shake-up was probably partly responsible for the high turnover that year, but transient families continued to be a big problem.
Last year, 23 of the school's 139 pupils left and roughly as many new faces turned up this year.
The sudden departures were usually linked to struggling families chasing work, which was often seasonal, while the arrivals included rent refugees, fleeing Auckland as the cost of housing rose.
Some children had been to 10 schools in as little as three years, she said. "You put a lot of time and energy into putting support around these kids and then boom, they move again."
New Zealand Principals' Federation president Phil Harding said pupil-churn was hugely frustrating for teachers, as it often made it difficult to reach children most in need. "It drives schools and teachers nuts."
He had taught one student who had attended eight schools in his first two years of school, as his mother shifted from house to house to escape debt. "I said to his mother, ‘You are going to condemn this child to failure if you keep doing this.' "
Children's Commissioner Russell Wills said it was clear that shifting schools had a huge impact on a child's development, particularly in primary school. Not only did they miss weeks of school while moving, but their learning was impaired as they eased into their new environment.
"The more transient a child, the less they learn, and the harder it is for teachers to get a handle on what they have learnt."
The higher level of transience in low-decile schools was not surprising, as deprived families were more likely to move for housing or work.
"Many of these transient families do not have a fixed abode. They are just staying with whanau for a while, until they have to move on again."
However, the rate of student turnover is improving, dropping steadily since the ministry started measuring it in 2010.
In 2010, the decile 2 Epuni School, in Lower Hutt, had the highest student turnover in the region, but principal Bunnie Willing said that had changed.
She said the school had worked hard to involve parents, building a 4000 sq m community garden and holding cooking and sewing classes for adults.
KIDS ON THE MOVE
The Ministry of Education measures "average student movement rate" as the number of children coming, going and even returning to a school, compared with the school roll in March of that same year.
At a decile 1 primary school last year, the average movement rate was 52.8 per cent.
At a decile 10 primary school it was 29.4 per cent. At a decile 1 secondary school it was 30.8 per cent.
At a decile 10 secondary school it was 4.8 per cent.
Overall, a decile 10 school had an average student movement rate of 24.8 per cent last year, while a decile 1 school's was 48.4 per cent.
- The Dominion Post