Two people were taken to hospital after several young men armed with bats and a machete stormed a Sydney high school, attacking students and teachers and leaving a trail of damage, police say.

A total of 18 children were treated by ambulance crews at the scene following the incident at Merrylands High School, in Sydney's west, about 9.45am (AEST) on Monday.

An Ambulance Service of NSW spokesman said one student was later admitted to Westmead Children's Hospital for treatment of a "minor facial injury".

A teacher was also taken to Westmead Hospital with bruising to the back of the head.

"We've assessed 18 school-aged children ... all having minor injuries, some lacerations, some bruising and they're obviously quite anxious about what has happened," the ambulance spokesman said.

The group of armed youths allegedly damaged classrooms, broke windows and assaulted students during the incident, police said.

The school and students were placed in "lockdown" - confined to their classrooms - as the incident unfolded.

"Police were called to the high school on Sherwood Road following reports a number of males had entered the school grounds," a NSW Police Force spokeswoman said earlier.

"They allegedly damaged a number of classrooms (and) a number of students received minor injuries.

"Some were assaulted by the group and others received injuries from the flying glass."

The spokeswoman said she could not comment on whether the five males were students, former students, or unrelated to the school.

Five young men were arrested shortly after police arrived at the scene. A crime scene has been established.

The NSW Department of Education said it was preparing a statement on the matter.

However, the department's regional director, Tom Murie, told Macquarie Radio he did not believe the arrested boys were students of Merrylands High School.

"At this stage we don't believe they're school students," Mr Murie said, adding there was no information about the alleged offenders' ages.

He said there was no warning of the attack and he praised the actions of staff and students who were taken by surprise.

"There are a number of students with minor injuries, although any injury is serious and there is one teacher who has also been treated by ambulance officers, but we believe they are all well apart from the obvious shock that such an incident would cause," Mr Murie said.

"It's my understanding that it was without warning and can I say that the principal ... acted in an outstanding manner and so did the staff and so did the students."

The boys have since been charged with 101 offences including assault, malicious damage and affray.

It was likely there were multiple charges of affray, assault and malicious damage for each of the five youths, hence the large number of total charges, a police spokesman said.

All five have been refused bail and will face Parramatta Children's Court today.