Charges are yet to be laid over a fire that tore through a building at Comet Bay College in Secret Harbour.
The fire started in the English and Humanities and Social Studies building at the high school on Thursday night.
It damaged several classrooms and forced the closure of the school on Friday.
Parents were also asked to keep their children home on Monday except for Year 11 ATAR students who are taking exams.
A 15-year-old boy was taken into custody but has since been released.
It's understood he is undergoing medical treatment.
The investigation is still ongoing.
It's not yet known when the school will reopen.
Mission Australia celebrates 25 years of supporting young people through Mandurah Reconnect program
'Mad Eagles fan' wins $6 million Lotto prize
Mandurah woman loses everything in house fire
Gift card scams surge past $1 million prompting urgent warning from Consumer Protection
Secret Harbour by-election attracts record 16 candidates
Major party candidates locked-in for looming Secret Harbour by-election
Comet Bay Primary to offer full-time kindy from 2027
North Dandalup crash claims life of driver
Date set for Mandurah's Town Ward Extraordinary Election