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A Christchurch Intermediate school opened as usual today, despite an arsonist's attempt to burn it down last night.

Police and Fire Service safety officers were back at Casebrook Intermediate School, in the northwest of the city, this morning investigating the blaze which is being treated as suspicious.

Firefighters were called to the school's flaming art room about 7.20pm.

A 14-year-old former student alerted the Fire Service to the blaze, after spotting black smoke billowing out of the room.