The popular Woden Valley school has been at capacity enrolments for some time – with at least one class having 37 students – and teachers have resorted to using their own staff room and library as classrooms.

The school was formally notified on Thursday by Education Minister Shane Rattenbury that a demountable would be installed for the start of term three and would be used to accommodate science and maths learning.

Mr Rattenbury accepted an invitation from the Parents and Citizens' Association to tour the school last month.

In Thursday's Canberra Times Garran was named as one of a growing number of schools struggling to cope with enrolments from in-area students, amid condemnation from education groups about a change to the way capacity enrolments were calculated.

Mr Rattenbury noted the decision to fund the demountable had been made by cabinet the previous week.

Mr Rattenbury said the new definition – which has the effect of adding hundreds of "new" places to school capacity that were previously quarantined for special education and non-mainstream classes – was a more accurate reflection of overall student numbers, and he said Garran was a prime example of a school that presented challenging capacity needs.

"We have put in place other measures [removing O'Malley students from the priority enrolment area] and our projections are that the school will see a reduction in enrolments over the next few years, but right now we need to accommodate a lot of students to deal with that short-term pressure," he said.

"We try and balance planning priorities with the needs of a school 10 or 20 years down the track."

Mr Rattenbury has also approved a one-off grant of $155,000 for Garran to replace a dangerously run-down pathway between the main school and pre-school.

The pathway has been an issue of concern for the school, with students, staff and families using it each day but finding it difficult to negotiate, particularly with prams or a wheelchair.

Despite a number of representations to the directorate last year, nothing was done and a teacher suffered a serious leg injury after falling on it.

Mr Rattenbury said he had been surprised by the state of the pathway after visiting the school.

He said it had been made precarious through the construction of a school hall through federal Labor's Building the Education Revolution program.

"They left an extraordinary drop away when they built the hall, and frankly it should have been fixed as part of the BER. So the job wasn't done properly, but we will fix it."

Opposition education spokesman Steve Doszpot welcomed the demountable and path remediation funding.

"This is a good result, but it does come after some years of trying to get a resolution," he said.

Mr Doszpot also questioned whether just one demountable would alleviate the school's crush, and noted Garran's increasing urban density and academic reputation suggested there would be no let-up in enrolments in the near future.