Police said the incident involving two boys happened just before 11am on Wednesday at Gilmore College in Orelia.
Paramedics rushed the injured boy to Royal Perth Hospital.
“A student is currently assisting police with their investigation,” police said in a statement.
“Inquiries into this matter are continuing.”
Gilmore College opened in January 2008 after the closure of Kwinana Senior High School.
In a note sent to parents, principal Rohan Smith described it as a “serious incident”.
“At this early stage, only minimal details are available, but what I can tell you is that one of our students has been taken to hospital and police are on site,” he said.
“I can assure you that all other students and staff are safe, and we will be putting in place support for any students who need it.
“Within the school, we have a team of support staff available to assist students, staff and family members.
“We will continue to provide this support as needed over the coming days and weeks.”
Originally published as Boy in hospital after school fight