SYDNEY • A teenage boy walked into a school classroom in western Sydney yesterday and allegedly stabbed a teacher and two students during morning roll call, before walking away without saying a word, a witness said.

A 16-year-old student was arrested in the carpark of a nearby supermarket shortly after the attack at the school in the suburb of Bonnyrigg.

A meat cleaver, two knives, a pair of scissors and two screwdrivers were found with a black sports bag not far from where the boy was arrested, The Sydney Morning Herald reported.

The boy was later charged over the attack, according to Australian Broadcasting Corp (ABC).

A 48-year-old female teacher was stabbed in the back, and two students - a 16-year-old boy and a 15-year-old girl - were stabbed in the shoulder.

All three victims were taken to hospital and were in stable condition, ABC reported.

"I didn't kill anyone. You have to tell my brother," said the suspect, who was wearing handcuffs and a school uniform, according to The Sydney Morning Herald.

Police and paramedics were called to the high school at around 8.45am after reports that a number of people had been stabbed.

A student who declined to be named said she was sitting in roll call in the school's science block when a boy entered the room.

"He just walked in and stabbed them, then walked out," she said.

She said the boy was supposed to be in roll call that morning. He did not say anything and most of the other students sat in shock.

Police Superintendent Peter Lennon said that there was no obvious motive for the attack.

"He is not known to the police or to the school principal in any fashion," The Sydney Morning Herald quoted him as saying.

"It has come as a surprise to everybody."

The boy was refused bail and will appear before a juvenile court today.

Police said the incident was isolated and not terrorism-related, ABC reported.