Memorabilia for Galston High School's 40th anniversary celebrations as well as archives and historic honour boards have been destroyed in a suspicious fire which has gutted the northwest Sydney school's library.
Police say there may have been a break-in at the library building before the fire.
The two-storey brick building went up in flames about 1am on Monday, causing more than $2 million worth of damage.
"It appears that there was a forced entry to a door," Detective Inspector Neil Higgins told AAP.
"We're still trying to work out if the damage to the door was caused through a break-in or by the fire."
Sniffer dogs will explore the charred ruins, searching for traces of accelerants that could have been used to start the fire.
Memorabilia for the school's 40th anniversary in October and honour boards have been destroyed, as well as books, computers and archives.
The school was closed on Monday and it's hoped it will reopen on Tuesday.
"It's a tragedy for the school," said Rural Fire Service (RFS) Commissioner Shane Fitzsimmons.
About 40 RFS firefighters responded to the blaze and crews were expected to remain at the scene for most of the day.
"Pretty much all of that building is extensively damaged. The brigades did some great work stopping the spread of the fire to adjoining buildings in the school," RFS inspector Peter Marshall told ABC radio.
The roof of the building collapsed during the fire and structural engineers were assessing the damage.
