A SCIENCE teacher tried to intervene when a student allegedly started stabbing his classmates before turning on her in a frenzied attack at a Sydney High School this morning.

Police received reports of a triple stabbing in the science block at Bonnyrigg High School, 36km west of the Sydney CBD, just after 8.45am this morning.

A 16-year-old male student allegedly walked into a classroom and stabbed a female student, 15 in the shoulder, a male student, 16 in the chest and a 48-year-old female teacher in the back.

The boy was charged with wounding with intent to murder and two counts of wounding a person with intent to cause grievous bodily harm.

He has been refused bail and will appear at a children’s court tomorrow.

The science teacher, Carolyn Cox, 48 was reportedly “opening the classroom” as the attack unfolded, just four days into the school term.

A woman whose younger sister witnessed the alleged stabbing said her sibling was badly shaken up by the incident.

The girl rang from the school to describe what had allegedly happened in the classroom.

“She just told me that someone in her roll call stabbed another kid and the teacher tried stopping it and (they) injured the teacher too,” the woman said via Facebook.

The 16-year-old boy in a school uniform was arrested in a carpark nearby after a knife, a meat cleaver, a Stanley knife and some scissors were allegedly dropped at the scene. He was chased by police into a neighbouring shopping complex, before he was handcuffed and escorted to Fairfield Police Station for questioning.

He allegedly said to police, “I didn’t kill anyone ... you have to tell my brother” Fairfax reported.

#BREAKING: There is a situation unfolding at Bonnyrigg High School this morning. Avoid the area. We will keep you updated as news comes in. — Fairfield Advance (@fairfieldadvanc) February 1, 2017

Police seize meat cleaver found in Bonnyrigg student's bag. Teen undergoing mental health check after stabbing spree pic.twitter.com/HA4uwkvm47 — Andrew Denney (@Andrew_Denney) February 2, 2017

Superintendent Peter Lennon told reporters the school was in lockdown when emergency services arrived on the scene.

“The female teacher was opening a class room as part of the school day (when) something has occurred,” Supt Lennon said.

“We’re not sure what it is that occurred that has led to the (multiple) stabbing.”

Police said the student accused of the triple stabbing was a local resident not previously known to police.

“He (the boy who was arrested) is not know to the police or to the school principal in any fashion,” Supt Lennon said.

“It has come as a surprise to everybody.

“This is not terrorism related.

“This is a local issue at a local school.”

The injured teacher and male student were transported to Liverpool Hospital. The female student was taken to Westmead Hospital. All three were in stable conditions.

“Their injuries are not believed to be life threatening,” police said in a statement.

One student, who asked to remain anonymous, told news.com.au “everyone was shaken”.

She said she was in roll-call in the school’s science block when a boy walked in and allegedly stabbed Ms Cox and two teens.

“Then (the accused boy) just walked out,” she said.

Principal Michael Bryce has reportedly addressed the school regarding the incident and ordered a “media ban”.

“In cases of serious incidents police put a blanket on all media,” a statement from the school read on its Facebook page.

“This incident is now in the hands of the police.”

Mr Bryce could not be contacted for comment this morning.

As the new education year started earlier this week Mr Bryce posted a video on the school’s Facebook page saying Bonnyrigg High was “one of the best schools in NSW”.

Fairfield police are investigating after a teacher & two students were stabbed at a school in Bonnyrigg - not life threatening — NSW Police (@nswpolice) February 1, 2017

A Department of Education spokesman refused to comment on the stabbing.

“It’s a police matter,” he said.

megan.palin@news.com.au