The fire damaged the walls of the storage room and a wall of the gymnasium where the two buildings connected. Mrs Kennett said fire crews extinguished the fire before further damage occurred. Mr Solomon said it would be "devastating" if the fire was confirmed as malicious.
"If it is arson it's a mindless and senseless act."
Mr Solomon said the gym was out of action yesterday and today.
But as school holidays started tomorrow, Mr Solomon said it gave the school a window of opportunity to organise repairs. He was unsure how much it would cost to repair the gym as he was waiting for engineers to look at the damage.
"That [repairs] will get under way as soon as possible," he said.
Detective Sergeant John Clayton said police would like to hear from anyone who may recognise the two women or anyone who saw suspicious activity around the school at the time of the fire. Police would also like to hear from anyone who is missing a wheelie bin.
Anyone who recognises the two women or who has information about the fire is asked to call Detective Sergeant John Clayton on 021 191 5827 or if they wish to remain anonymous they can call Crimestoppers on 0800 555 111.