THE arson squad will launch an investigation after a suspicious fire badly damaged buildings at a high school in Perth's south.

Emergency services were called to the blaze at Atwell College at 2.45am on Monday morning.

Fire crews have now left the scene after extinguishing the blaze but police remain at the scene ahead of the arson squad arriving later this morning.

Camera Icon Credit: PerthNow

The cause of the fire is unknown but it is believed it started in an air conditioner.

It's believed one of the buildings that suffered damage was the school's library.

Students are attending classes as normal this morning.