The fire service said they were called shortly before 8pm on Tuesday.
A single storey extension of the school is '100 per cent involved in fire' according to the service.
Two breathing apparatus, one jet, one hose reel and three positive pressure ventilation fans are in use to prevent spread to adjacent building, the fire service said.
A total of ten fire engines and one aerial ladder platform is in attendance.
"Advice to local residents is to keep doors and windows closed", the fire service said.