Wyndham Crime Investigation Unit detectives are appealing for witnesses to a suspicious fire in Truganina that occurred on 3 April.
Passers-by contacted emergency services when they observed fire burning in a portable classroom at a Derrimut Road school about 9.55am.
The school was unattended at the time and the campus had been unoccupied for approximately three months prior.
No one was injured during the blaze.
Anyone with information is urged to contact Crime Stoppers on 1800 333 000 or submit a confidential report online at www.crimestoppersvic.com.au