Shifting schools may set children back by about half a year in their learning, a new report says.
The new Ministry of Education analysis also shows that children achieve in line with the curriculum expectations in the first few years of school, but their progress slows as they get older and they are falling below expectations by intermediate and early high school years.
The analysis, based on electronic Assessment Tools for Teaching and Learning (e-asTTle) tests, shows student achievement in reading, writing and maths varied hugely.
For example, in Year 8 maths, the top 5 per cent of students in high-decile schools achieved almost 1700 points on the e-asTTle scale, compared with about 1350 points for the lowest 5 per cent in low-decile schools - a gap equivalent to about seven years' progress for the average child.
On average, students in Years 4 to 10 are expected to move up by about 50 points a year in maths and reading, and by about 58 points a year in writing.
Actual progress fell below this average in maths, but was much the same across all deciles, showing that schools neither narrowed nor widened the gaps that children entered schools with.
But students who changed schools during Year 8 averaged 27 points behind students who stayed in the same school in maths, 23 points behind in reading and 28 points behind in writing - about a half a year's expected progress in each subject.
Individual schools have found similar effects.
Sylvia Park School principal Barbara Ala'alatoa said last year that 77 per cent of her students who stayed in the school all year achieved national standards for reading in 2016, compared with only 72 per cent when new students were included.
The report cautions that the gap may reflect other factors besides changing schools.
"It is possible that the characteristics and learning abilities of students who change schools differ to those who remain in the same school," it says.
"For example, if a child has behavioural issues or learning challenges at one school, that might precipitate a move to a new school. If this was a common reason for changing schools, then it would lead to lower observed achievement levels of children who changed schools."
On average, progress in all three subjects is the lowest in Year 9 when almost all students change schools to go to secondary school.
Massey University education professor John O'Neill said the data showed a need to manage transitions between schools better.
"One of the problems faced by secondary schools is that they have multiple feeder schools and spend a lot of time trying to sort out where children actually are, as opposed to where they are reported to be," he said.
"If you add that achievement challenge to the major challenges to self-esteem, wellbeing and confidence and all the other things that go with moving from primary to secondary school, such as different patterns of organisation, then it's not surprising there would be some loss of progress."
He said "communities of learning", which now group most high schools with their contributing primary and intermediate schools, might help schools to work together better.
Dr Jenny Poskitt, who leads a new NZ Assessment Institute, said secondary school students actually made faster progress thanks to specialist subject teachers, once they adjusted to the change.
"There is a trend across all data sets for a dip in the transition from Year 8 to Year 9," she said.
"It's usually about a term's difference, and then the rate of progress increases slightly so by the end of Year 9 they have caught up."
The report shows average achievement in reading, writing and maths did not change significantly during the period of national standards from 2011 to 2016.
Graphs show slight improvements in reading and maths in Years 7 to 10, but the only significant improvement was for writing in the first two years of secondary school.