Firefighters were called to Silverstream High School in Sonderend Road, Manenberg, on Saturday just before 12.30pm.
It is unclear how the blaze began.
Rumours spread at the scene that paper that had been placed inside one of the classrooms was set alight.
The Western Cape Education Department (WCED) has since deployed security to the school to guard the grounds.
The WCED says despite the extensive damage, schooling will continue as normal today.
Police spokesperson Lieutenant-Colonel André Traut confirms a case of arson has been opened for investigation: “A case of arson is being investigated after four classrooms were damaged by fire at Silverstream High School in Manenberg.
“No one was injured and no one has been arrested as yet.”
But Jessica Shelver, spokesperson for Education MEC Debbie Schäfer, says according to their information, three and not four classrooms were damaged.
She explains the school had enough classrooms to accommodate all pupils.
“There is reportedly extensive fire damage to the building and three classrooms were destroyed in the fire,” says Shelver.
“The cause of the fire is not known at this stage, but the City’s Fire and Rescue Services are investigating.
“We are awaiting a report from the City regarding the full extent of damages.
“The school reportedly has enough classrooms to function normally and the learners’ education will not be negatively impacted.
“There were extra lessons taking place at the time. Learners and staff were evacuated.”
“The WCED Safe Schools has deployed emergency security to the school.”
Tracey Whittaker, assistant media spokesperson for the City of Cape Town Fire and Rescue Service, says they have yet to determine how the fire started.
“The City’s Fire and Rescue Service responded to Silverstream High School in Sonderend Road, Manenberg at approximately 12.23pm on Saturday,” she says.
“Six firefighting vehicles responded with 20 firefighters.
“Three classrooms were destroyed by the fire. There were no injuries or fatalities reported.
“The cause of the fire is undetermined at this stage.”