Analysis of National Assessment Program - Literacy and Numeracy withdrawal rates conducted by Save Our Schools shows ACT parents withdrew 4 per cent of year 3 students from the numeracy test this year, compared with just 0.8 per cent in 2008, when NAPLAN began.

A total of 579 students out of 17,500 sitting the numeracy test across years 3, 5, 7 and 9 were withdrawn in the ACT this year. The national rate rose to 2.9 per cent, compared with 0.5 per cent in 2008.

Withdrawals require parents or carers to specify their child is not to sit the NAPLAN test. They differ from exemptions, which are commonly given to students with disabilities, who have limited English language skills or who are sick on the day.

Withdrawal rates were highlighted as an issue of concern last week by members of the Council of Australian Government Reform Council, which warned in its national education agreement report that changes in the participation rate could affect achievement rates.

Research earlier commissioned by the council showed children who tend not to participate in NAPLAN were often lower scoring students and there was debate over whether schools were encouraging poorer performers to withdraw from testing in order to raise their average results.

Separate analysis of literacy rates has not been undertaken, but are expected to show withdrawal rates similar to those for numeracy.

The rising rate of withdrawals is not just in the ACT. All year levels across all states and territories are showing increased withdrawals - the next largest increases are in Queensland, South Australia and Victoria.

The Ministerial Council for Education has asked the Australian Curriculum and Assessment and Reporting Authority to report on participation rate trends.

Save Our Schools convener Trevor Cobbold said: ''It's not clear whether the increase is due to increasing parent concerns about NAPLAN, increasing rorting of school results or a combination of factors.

''Certainly, more and more parents are becoming aware that NAPLAN is not compulsory despite the efforts of education authorities to suggest they are mandatory.

''Certainly, schools are under tremendous pressure to improve their results and there is anecdotal evidence of schools encouraging parents of lower achieving students to withdraw them from the tests or keep them home on test days.''

Mr Cobbold, a former Productivity Commission economist, said that although the proportion of students withdrawing was still small, the rapid growth in withdrawals ''poses a threat to the reliability of NAPLAN results for inter-school comparisons, inter-jurisdictional comparisons and trends in indicators of student achievement.''

In the ACT, the proportion of year 3 students withdrawing from the numeracy tests rose from 0.8 per cent in 2008 to 4 per cent this year.

In South Australia, the proportion withdrawn increased from 0.6 per cent to 3.3 per cent; in Queensland it rose from 0.3 per cent to 2.4 per cent and in Victoria from 0.1 per cent to 2.4 per cent.

Nationally, it rose from 0.5 per cent to 1.9 per cent, while the smallest increase was in NSW, where the proportion rose from 0.8 per cent to 1 per cent.

There was also a smaller but significant increase in years 5, 7 and 9 students withdrawing from numeracy testing. At year 9 level, the national rate of withdrawal rose from 0.3 per cent in 2008 to 1.4 per cent this year.

While the trend is for an increasing number of students to withdraw from NAPLAN, there has been little change in the proportion of students absent or exempt since 2008.

ACT Education Minister Joy Burch said on Thursday she would work with the Education Directorate to increase participation rates across the ACT.

''The integrity of the testing regime is crucial. NAPLAN test results are critical to understanding how students across the ACT are performing in literacy and numeracy and assists systems and schools to focus on the needs of students,'' she said.

''While withdrawal rates have increased, the numbers remain low and do not affect the validity of the NAPLAN data. More than 17,500 students sat the numeracy test in the ACT, while only 579 were withdrawn.''

Federal Schools Minister Peter Garrett said: ''While the tests are not compulsory, all students are encouraged to participate. By not taking part, students and their parents miss out on the opportunity to see where they are doing well and what areas they may need work on.''

The Australian Education Union's ACT branch said there was an increasing awareness among parents that they can withdraw their children from NAPLAN tests - made particularly clear on the ACT Education Directorate's website.

The union's branch secretary, Glenn Fowler, said ''parents increasingly understand that NAPLAN is one measure among many and that one learns most about their child's progress when they talk to the teacher and receive feedback on tasks set directly by that teacher''.

''The trend towards withdrawal of students reflects an increasing understanding of the limitations and flawed use of the NAPLAN tests and reduces the reliability of the data with each passing year,'' he said.