Roads nearby were closed to allow emergency services access to the site, where they remained throughout the following day.
They have since reopened.
Detective Constable Stuart Hache, of Aberdeen CID, said: “Thankfully no one has been injured as a result of the fire however we are now treating the fire as wilful.
“We are still appealing for anyone who has information and are especially keen to speak to a group of youths who were seen in the vicinity at the time of the outbreak of fire.”
Anyone with information is asked to contact police on 101.
Police treating Aberdeen former school blaze as deliberate