Almost 20 per cent of Canberra public schools are full or almost full, while others have just a third of the students they could accommodate, new government data shows.
For the first time, the ACT has included capacity figures in its annual school census report, revealing an uneven spread of enrolments in both north and south.
While 15 schools were close to bursting at between 102 and 90 per cent capacity as of February, a dozen others were less than half full.
A school's capacity is now measured on a "case by case basis" after a review earlier this year, according to a spokeswoman for the education directorate. While the figure may still include some specialist rooms such as science spaces alongside traditional classrooms, other areas like kitchens and art rooms that would cost too much to convert were no longer counted in capacity for primary schools.
"[The] review did not result in any increases in capacity, however has resulted in some decreases in capacity..[for] 17 of the primary schools," the spokeswoman said.
Preschool enrolments are not counted in capacity metrics.
According to the report, Lyneham High is the most stretched school in the territory, recording 31 students over capacity as schools in Canberra's north struggle to cope with accelerating population growth. Hawker and Fraser primary schools were also just shy of full.
But across the lake, a number of schools in Canberra's south are also filling up fast. The Canberra College was at 99.8 per cent capacity, while Telopea Park had grown to more than 93 per cent.
The directorate spokeswoman said the ACT government was not concerned about schools at 90 per cent capacity or more as "we know enrolments...go through natural cycles of growth and decline".
"Every child who seeks a place has a place in their neighbourhood school," she said.
"A school at 90% capacity demonstrates that we are making great use of that school's capacity."
In the Molonglo Valley, Charles Weston School, which opened in 2016, was still only 32 per cent full but saw its enrolments climb by more than 46 per cent between 2017 and 2018.
Maribyrnong Primary, the University of Canberra High School Kaleen and Neville Bonner Primary also clocked rapid growth between this year and last, while the University of Canberra Senior Secondary College Lake Ginninderra and Lyons Early Childhood School appeared to be shrinking the fastest in the same period.
The snapshot also revealed Canberra's public schools are continuing to claw back enrolments from the non-government sector. Almost two thirds of the 77,115 students in the territory now attend a government school, and enrolments have surged by more than 13 per cent over the past four years.
Non-government enrolments also rose in that time, but by just under 3 per cent.
As well as adding temporary classrooms and announcing new schools in areas of rising demand, the ACT has recently changed the rules on cross-border enrolment to limit the number of schools NSW families can send their children.
The move came after some Canberra schools grappled with capacity issues in 2017, forced to turn libraries into classrooms and else asking parents not to come to school assemblies due to space constraints.
Priority enrolment areas were also redrawn for Canberra College as well as North Ainslie, Garran, Mawson and Aranda primary schools to "reduce enrolment pressures", the directorate confirmed. All five schools recorded a rise this year.
As of February, about 300 students across the territory were home-schooled. Another 3311 were accessing special needs programs, with the majority learning in the public system.
Capacity figures for non-government schools were not included in the 2018 census.