The St Clair High School community is devastated by a fire that has gutted about 30 classrooms.
Principal Chris Presland said early media reports the blaze started in the science labs were inaccurate, with the cause yet to be determined by forensic investigators.
"People are quite devastated and shocked," he said.
"This is a school with a very strong community spirit. Lots of community and youth organisations use the school grounds.
"At this stage the cause of the fire is unknown and there is no indication that the fire was deliberately lit."
Mr Presland said the focus was now to ensure classes could resume on July 14 — the first day of term 3 — with demountable classrooms to house the students.
A Department of Education spokesman said priority would be given to minimising the disruption to year 12 studies.
Emergency services were called to Endeavour Avenue about 1.30am on Sunday to find the south-west side of the school well alight.
Twenty-five fire trucks fought the blaze, which seriously damaged science blocks, the canteen and an estimated 30 classrooms.
High winds hampered the firefight so seriously that crews had to temporarily withdraw from the scene.
Spot fires within the complex also raised fears the flames could spread, but firefighters managed to contain them.
Concerns about possible asbestos contamination also prevented them from returning to the scene.
The fire closed Endeavour Avenue for several hours and traffic diversions were put in place.
St Marys police are investigating but said the causes of the fire were still not known. Officers doorknocked the area and will continue to do so while inquiries continue.
Rick Scavera said his daughter, a year 9 student at the school, cared that the school had been badly burnt but he believed the impact wouldn't be felt by students for two weeks.
"It really won't affect her until she goes back to school after the holidays," he said.
"Hopefully the government or the education department can fix up the mess.
"Losing 20 years of library resources is a big blow."
Fire investigators are going over the scene, using special fire dogs to sniff out elusive evidence.
Anyone with information is urged to contact St Marys police on 9677 7501 or Crime Stoppers on 1800 333 000.