St John Ambulance received an emergency call at 10am and took the boy to Royal Perth Hospital.
The circumstances surrounding the fall are not yet known.
Headmaster Stuart Meade confirmed the incident occurred.
‘Just after 10am one of our senior school students fell from the second storey of one of our classroom blocks,’ he said.
In a statement, the school said all students had been offered counselling and support and their parents had been notified. A Royal Perth Hospital spokesperson said the boy was in a serious but stable condition.