The warning comes amid concerns that ''conscientious objectors'' pulling their children out of NAPLAN tests could distort information made available to parents on the government's My School website.

More than 1 million students in years 3, 5, 7 and 9 across Australia will today start NAPLAN testing for numeracy and literacy.

Last month, a group of educators launched the ''Say no to NAPLAN'' campaign to encourage parents to boycott the tests, which are now in their fifth year.

The group is publicising the previously little-known fact that the tests are not compulsory and parents have the right to withdraw their children on the grounds of philosophical objections or religious beliefs.

The NAPLAN 2012 Information for Parents brochure, for example, only mentions that students can be exempted from the tests if they have a significant intellectual disability or have recently arrived in Australia and come from a non-English-speaking background.

More than 100 academics from universities around Australia have signed a letter of support for the Say no to NAPLAN campaign, saying the tests have little merit and are being misused for political agendas.

''Pressure on teachers and children to perform well on NAPLAN tests is narrowing the curriculum and eroding time in classrooms for quality teaching and learning activities,'' they say.

But Australian Curriculum Assessment and Reporting Authority general manager Peter Adams said state and federal governments expected students to sit the tests.

"Parents do have the right to withdraw their children from the tests, but we emphasise that principals are not to actively encourage students not to participate,'' he said.

''We would consider that quite inappropriate for obvious reasons.''

Mr Adams said NAPLAN participation rates were not declining, with the average Australian participation rate 95.9 per cent.

But Brian Caldwell, an educational research professorial fellow at Melbourne University, said participation rates were likely to fall as parents became more aware of concerns about the way NAPLAN data was used and the fact the tests were not compulsory. ''I would expect the number of withdrawals are likely to increase,'' he said.

''This would reduce the validity of what is being reported on My School, which is solely dependent on getting high rates of participation.''

Professor Caldwell said lower test participation rates would limit the validity of comparisons between NAPLAN results achieved by schools and states.

The NAPLAN results published on the My School website for Spensley Street Primary School in Clifton Hill, Victoria, for example, are skewed because only 14 per cent of its 367 students sat the test last year.

According to the website, the school is performing substantially below similar schools in almost every category - a ranking that would appear at odds with the socio-economic profile of the families who send their children there.

Principal Anne Nelson said Spensley Street Primary believed assessment was very important and the way students were assessed was communicated to parents.

''This has created a culture where parents, in looking at all ways to assess students, don't see NAPLAN as a very important part of that. Our school attitude to NAPLAN is on the record and I think when parents choose this school for their children they understand. We don't say upfront we don't do NAPLAN because it's not true.''

Federal School Education Minister Peter Garrett said parents understood why NAPLAN was important and valued the fact that they received detailed information on how their children were progressing in vital literacy and numeracy skills.

"Participation rates in NAPLAN have always been very high and I expect that to remain the case for this year's tests,'' Mr Garrett said. "All students are encouraged to take part in the tests except in circumstances where their participation would be inappropriate.''