At one of Canberra's most packed schools, there aren't many classrooms to spare, but neither are there students working in corridors.

Principal Rob Emanuel admits timetabling is a daily juggling act in a school now 31 students over capacity, and he's regularly forced to turn away families who live out of the school's priority enrolment area. But, day-to-day, he reports Lyneham High is bearing the extra load well.

According to the latest government data, the school is one of 15 across the territory now more than 90 per cent full as enrolments in the public system surge.

The ACT government has revealed it will invest in school expansion in this year's budget, with $18.8 million set aside to upgrade and replace aging buildings at Campbell Primary School.

A new school is also planned for the growing Molongo Valley, after the Charles Weston school came to the area in 2016.

The $47 million P-6 school will open in 2021 in Denman Prospect, with enough room for about 600 students and 44 equivalent preschoolers.

ACT Education Minister Yvette Berry said the government would also undertake early planning and feasability work for a future 7-10 campus "that will come online as Molonglo grows".

Deputy Director-General at the education directorate Meg Brighton said there were guidelines for the number of students that could be in a classroom per square metre for new school builds, but no set rules for existing buildings.

Principals were more guided by student teacher ratios, which she noted were still among the lowest in the country.

Ms Brighton said the Australian National University was helping the directorate revise its forecasts for future school demand, including around capacity.

At Lyneham, Mr Emanuel said there were advantages to being big.

"We can offer so many electives," he said. "I can have 10 language classes going."

While teachers frequently booked the library for both classes and staff meetings, the facility hadn't been counted in the government's capacity figures.

Ms Brighton said principals were encouraged to monitor what they were using spaces for as student learning evolved, to see if they could be better used in other ways.

"Fifteen years ago building computer labs was all the rage, now we have Chromebooks and we don't need nearly as many," Mr Emanuel said.