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.
Armed man arrested in Port Kennedy
Police seek witnesses to serious crash in Pinjarra
Local artist Emma Blyth inspires community to raise $10k for Mandurah kids facing hunger
Hunt continues for Baldivis ATM thieves
Channel 7 Mandurah Crab Fest a cracking success
Giant puppets, illuminated installations to descend on Baldivis Sports Complex
Man fined nearly $2k for undersize crab haul near Coodanup
Australia Post reveals Mandurah residents receive highest volumes of parcels
Safety Bay man charged over South West pursuit