Principals are worried that primary schools in poor areas will be made to look bad when national standards in reading, writing and maths are introduced next week.

All schools will have to test pupils against the standards from Year 1 to Year 8, the last year of intermediate school.

They will also have to report to parents on each child's progress twice a year and send results for all their pupils to the Ministry of Education by 2012.

Principals said they believed the new system would gradually overcome starting glitches, as most schools already tested students and used the results to improve children's learning.

But they questioned how the information would be presented when schools sent it to the ministry and journalists compiled it to make league tables showing school rankings.

"If schools like ours have their value-added (results) displayed, we will look good, and it won't be difficult to show it because we do it already," said Pt England School principal Russell Burt.

"If all that is displayed is the level of achievement, then schools like ours will be disadvantaged - it's as basic as that."

Mr Burt said pupils from the country's poorest families at decile 1 schools like his always started well behind the national average.

Typically they scored in the bottom 10 per cent for reading ability, putting them well out of reach of the national standard for several years.

Yet many low-decile schools improved their pupils' performance against the national average and sometimes lifted them to catch up with the average by Year 6 (10 to 11-year-olds).

"We can prove quite extraordinary value-added - in fact what we can prove is that our students have done one and a half years to everybody else's one."

Murrays Bay Primary principal Ken Pemberton agreed, saying decile 10 schools like his on the North Shore took children with good pre-school education and strong home support, which gave them a huge head start.

In lower decile schools it was almost the opposite.

No matter how hard the teachers worked it took years to make noticeable improvements.

Ponsonby Primary principal Anne Malcolm believed the standards should have been trialled in a smaller number of schools instead of the whole country at such short notice.