Police were called to the incident shortly after 5pm and are at the scene to assist with traffic control and liaise with fire staff.
A post on the school's Facebook page said the fire was in the hall and store room.
"No one was hurt. There is damage which is being assessed. Could you please avoid coming to school on Saturday night or Sunday as we need to ensure the site is safe and having sightseers will not help. We will send a further update once we have more information to share," the post said.