A CLASSROOM of Year 1 students has escaped injury after a runaway truck crashed into a Perth primary school, leaving the driver injured and trapped for about an hour.

There were 22 students in the Clifton Hills Primary School classroom in the southeastern suburb of Kelmscott when the garbage truck left the road, rolled down a slope and smashed into the building just before 9.30am on Thursday.

A nearby resident said he saw the driver leaning in through the open door and laying across the front seat, trying to apply the brake.

"It was on its way and I thought 'nothing's going to stop this'. Then by the time I got out the door, it was in the school," he told Perth radio station 6PR.

Emergency services freed the driver, who had non-life threatening injuries, and he was taken by ambulance to hospital wearing a neck brace.

An Education Department spokeswoman confirmed the students and teacher were safely evacuated to another part of the school.

She said the whole school block, not just the classroom, was evacuated as a precaution while a structural assessment was carried out.

The spokeswoman said counsellors were attending the school to support any shaken-up students, and parents were being contacted to advise them what had happened.

Power lines were damaged so police cordoned off the street and advised people in the area to be cautious.