SYDNEY, Australia — Five teenagers brandishing baseball bats and machetes rampaged through a suburban Sydney school yesterday and hit a teacher over the head, police said. Eighteen students were treated for minor injuries.

The drama unfolded as hundreds attended an assembly in an outdoor area of Merrylands High School. As the attackers moved in, teachers rushed the students back to class, where they sought refuge behind locked doors, under desks, even in a cupboard.

“I find it very difficult to believe the brazenness of how they entered the school,” Police Detective Inspector Jim Stewart said. Such school violence is rare in Australia.

The teenagers, between the ages of 14 and 16, were arrested and will appear in court today to face more than 100 charges, police said. They have been charged with offenses including assault and malicious damage and could face up to seven years in prison if convicted.

Police refused to say whether the teenagers were students at the school.