Four recycling bins were set on fire at Melrose High School in Pearce on Saturday night.
Police say the fire, which caused minimal damage at the rear of a building on Beasley Street, was suspicious and they are investigating.
"ACT Fire and Rescue attended and extinguished the blaze, which was located in four cardboard recycling bins against an external wall of the school building. There was minimal damage to the inside of the building," police said in a statement.
"AFP Forensic Services attended to examine the area and investigations are ongoing."
Police ask anyone who saw anything suspicious in the area about 9.30pm or who knows anything about the fires to contact Crime Stoppers on 1800 333 000, or at act.crimestoppers.com.au. Information can be provided anonymously.