The incident happened at 11.15am local time at Willetton Senior High School.
The student allegedly fled after the incident, involving a Year 8 co-ordinator in her 50s.
The woman’s injuries are non-life threatening and she’s in a stable condition in hospital.
The stabbing happened in the student services block, where children go for support and counselling.
One student, Jazmine Lin Wooden, said she saw blood on the floor.
“She’s my brother’s year coordinator, she’s always been nice, she’s always been one of the teachers you can talk to so I can’t really understand what’s happened,” she said.
“I messaged my sister and she was like ‘yeah, someone got stabbed’. I’m scared. The teacher said to lock the door and everything.”
An announcement was made over the PA for staff to keep children inside classrooms for about half an hour.
“It was so scary because I don’t think any of the teachers believed it,” another student said.
Surrounding primary schools Burrendah and Castlereagh were closed as a precaution.
Parents were sent an email.
WA Police confirmed they are investigating.
“A female teacher was assaulted by a student,” they said in a statement.
“During the incident the teacher received a minor wound and was taken to Fiona Stanley Hospital for treatment.
“Police are continuing their inquiries and are attempting to locate the student who has left the school grounds.”