Two nine-year-olds are believed to have started a fire at Drouin Primary School on Saturday afternoon.
Police said they had identified two boys, both of Drouin, in relation to the incident which occurred at 1.05pm.
They believe sheeting was torn off a window on the second level of the grade five building before an unknown item, which had been ignited, was thrown through the window.
The CFA attended and contained the fire to a bookcase.