Four fire crews and a command unit are battling a blaze in a classroom at Redwood Primary School in Christchurch.

Southern Fire Service shift manager Daryl Ball said they were alerted at 11pm by members of the public who reported seeing smoke and fire coming from the school.

"On arrival our guys found a classroom damaged by an externally started fire. There was damaged to walls."

Three appliances are still at the scene.

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The cause of the fire is not yet known.

Today marked the final day of the school year.