Since 2015, 675 have been installed, according to documents released to the Labor opposition under a freedom of information request. There are now 4665 demountables in the state's 2211 schools. Under department policy they are supposed to be used for a maximum of seven years. But 1462 – just under a third – of the structures have been in place for a decade or more. Labor education spokesman Jihad Dib said, "it's staggering that students are sitting in over 1450 demountables that have been in the same place for over 10 years. What is meant to be a short-term solution is simply becoming part of the furniture."

A Herald analysis reveals a wide gap between Sydney's east and the rest of the city, with students in the north, west and south-west twice as likely to be in demountable classrooms as those in the east. There are 3.7 demountables per 1000 students in the city's east, compared with 7.4 per 1000 in the north, 7.3 per 1000 in the west and 7.1 per 1000 in the south-west. Only two of the 228 schools in Sydney's east have 15 or more demountables, compared with 14 out of 245 schools in Sydney's west – the worst-affected region. Cherrybrook Technology High School (the state's largest school, with 1960 students) has 33 demountables, the most of any school in NSW. It is closely followed by public primary schools Carlingford West, Girraween and Matthew Pearce in Sydney's north-western suburbs (all with 28) and Beverly Hills Girls High (27). The average school in greater Sydney has three demountables.

It's not just older schools that are bursting at the seams – Dalmeny Public in Prestons opened in 2003, but now has 18 demountables for a student population double what the school was built for. "I just don't think they estimated the growth of the area," said Rebecca Singh, a parent at the school. She said about a quarter of the school's playground had been given over to demountables. Maria Rallis​, president of the P&C at McCallums Hill Public in Canterbury said the school's three demountables took up "precious playground space". "Year by year, children at our school are losing space for physical play which is vital for helping combat childhood obesity," she said.

"The students are devastated whenever they see a new one arrive as they're always placed in a prime position of our paddock, on flat and level ground, taking the spot where soccer games would normally take place at lunchtime." Mr Stokes conceded that demountables "devouring" playground space was "not sustainable", but said environmentally designed, two-storey, modular classrooms were an important part of the solution to boost school capacity. "I'm not pretending [demountables] are going to go away," he said. "Of course we will always need flexibility around classrooms in our schools ... but the problem has been with the designs in the past. We've designed demountables as a movable building but then left them there for 40 years. "If we reconceptualise them – design buildings that can be permanent but with the capacity to move them if necessary – I think it changes the way we look at them." Modular classroom blocks of two, four or six classrooms are cheaper than permanent buildings and will be used where enrolment growth is projected to last longer than seven years, he said.

The NSW school system is undergoing an unprecedented student boom, with 7200 additional classrooms needed in the next 15 years. A report on classroom capacity by the Auditor-General last week commended the department's recent plan to accommodate the booming student population, but pointed out the government has not committed to fully fund it. Mr Stokes said he could not comment on the government's intentions for the school infrastructure plan under the budget as that was a matter for the Treasurer, but said he accepted the findings of the Auditor-General's report that more money was needed. The report, Planning for School Infrastructure was critical of the long-term use of demountables but said they were a key part of the department's growth plan to meet capacity at a reasonable cost. A spokesman for the department said demountable classrooms are airconditioned and maintained regularly; and were "regularly replaced with permanent facilities through the department's capital works programs".

The figures show 165 demountables were removed from schools in 2016.