More WA children risk becoming overweight because of decreased physical activity as school sports ovals and play spaces make way for extra classrooms.
An analysis of the physical activity of 408 Year 6 students across WA found those at schools with larger grassed areas were the most active during breaks.
The report, published today in the Australian and New Zealand Journal of Public Health, warns that putting transportable classrooms on grassed surfaces to accommodate more students "may have a negative impact on children's physical activity".
The study, done in 2005, found a quarter of children surveyed were overweight or obese.
Report author Karen Martin, from the University of WA's school of population health, said these rates could increase because of the dwindling space for sports and physical activity. "Not only are you decreasing the area of school grounds, but increasing the number of children, so the space per child becomes even less," Dr Martin said.
"We're mollycoddling children by giving them more inside play when we should be encouraging them to be outside."
The study found that children participated in an extra 4½ minutes of physical activity per day for every 100sqm of grass at the school.
WA Primary Principals Association president Stephen Breen said some schools had lost play space to demountable classrooms.
"There are some schools that because of the growing population have no full oval there," he said. "That is manageable - teachers can walk students down to a community oval. But I would hope that planners are building schools that would look to maximise student activity."
Education Department executive director of infrastructure John Fischer said the 2800 transportable buildings in use around WA allowed the department to respond quickly to fluctuations in enrolments. "All schools are provided with an oval and, in primary schools, specific play areas are allocated," he said.
Leanne Bowdidge's son Matthew, 13, a student at Ballajura Community College, enjoys skateboarding in his spare time.
She said ovals should be for children to play on, not for temporary classrooms.