A group of teenagers armed with baseball bats and machetes attacked Australian teachers and students on Monday, injuring 18 people and forcing a high school in Sydney to be locked down, police said.

Five youths barged into the morning assembly at Merrylands High School in south-west Sydney, witnesses said, grabbing students before starting to smash windows at the school.

The school then locked its students in classrooms as the youths, who were not students, went through two school buildings, smashing more windows and equipment, police said.

"They caused a considerable amount of damage," a police spokesman told reporters. "There have been some assaults. But there are no serious injuries as a result of those assaults."

Attacks on schools are rare in Australia, where schoolyard security is usually low-key but south-western Sydney has a growing youth-gang problem.

Ambulance officers treated 18 people for minor injuries, including cuts from shattered glass, and shock, but one student and one teacher were taken to hospital with minor facial injuries and severe bruising.

Dozens of distressed parents rushed to the school, some in tears, police said.

Police arrested five boys, aged 14 to 16, after the attack.