Police allege the girls, aged 13 and 14, exchanged messages online for more than two weeks which related to “talking about and planning to kill a teacher.”

The 13-year-old allegedly brought a knife to Willetton Senior High School on Monday which she provided to the other girl.

It is alleged the 14-year-old used the knife to stab the teacher, who suffered a one centimetre wound to her left armpit and was taken to Fiona Stanley Hospital for treatment.

The 14-year-old fled but was found by police in Willetton later that day. The other girl was taken into custody when officers arrived at the school.

State School Teachers’ Union of WA president Pat Byrne said she was deeply disturbed by the incident.

“Every teacher should be able to go to work confident that they will be safe from harm,” she said.

“This incident is a reminder of the dangers teachers face and the need for constant improvements around safety in schools.”

Worried parents took to social media after receiving text messages from their children who were placed in lockdown at the school.

“It was a little worrying getting a text from my daughter this morning of her in a classroom, with all the lights off, having to keep quiet,” one mother wrote.

“Definitely an experience for both my kids. Hopefully one they don’t have to go through again.”

Both girls were charged with attempting unlawfully to kill. They are due to face Perth Children’s Court on Tuesday.