Police believe a group of young people may be behind a series of tree fires near schools in the bushfire-prone Dandenong Ranges, east of Melbourne.
In the latest blaze, a tree was set alight near the Upwey High School about 6:30am (AEDT).
Up to 20 tree fires have been started near schools at Upwey and Tecoma since July.
Police are yet to determine a motive for the attacks.
Police say a group of youths was spotted near one fire, but they have no suspects at this stage.
Detective Senior Sergeant Jeff Maher, from the arson squad, says police are doing all they can to find the culprits.
"We had a group of six youths seen in silhouette in one of the fires in the series, but at this stage we haven't firmed up a suspect," he said.
"I can tell you that the arson squad assisting the Yarra Ranges police will be pulling out all stops to catch this person."
He says there is a big risk to the community as the weather gets drier and hotter.
"In the next coming months the scrub will dry out and these fires pose a public safety threat in relation to the scrub around there - the area is densely populated," he said.
Detective Senior Sergeant Maher says the culprits will face severe punishment.
"I think the judiciary would frown upon this sort of behaviour and the community would frown on this sort of behaviour," he said.
"The penalties are very substantial - they go into many years' imprisonment."