The Kindergarten classroom and canteen are now out of operation, according to principal Jen Honner.

Ms Honner provided a statement on Wednesday afternoon, where she said that no buildings near the scene of the fire were able to be accessed for a while.

"I have been advised by the CSO (Catholic Schools Office) that the best course of action is to ask Kindergarten students to stay at home until Monday. I realise this may cause problems so if you are absolutely unable to keep your kinder child at home we will accommodate them in Year 1," she said.

She said that the most important thing was that nobody was hurt, and that the event had "shaken" the school community.

"We plan to get back to 'normal' as [soon as] possible," Ms Honner said.

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Firefighters spent much of the afternoon tackling the blaze that broke out about 3.40pm. Flames were seen leaping from the windows and thick black smoke started billowing into the air.

Due to the ferocity of the fire, firefighters in breathing apparatus attacked the fire in a defensive mode from outside of the school building.

Onlookers told ACM they had ash rain upon them as the crews worked to contain the blaze, and stop it from spreading.

Police cordoned off Little Bloomfield Street and Henry Street in the emergency, and NSW Ambulance, three Fire and Rescue NSW crews as well as two Gunnedah RFS crews remained at the scene late yesterday.

A Fire and Rescue NSW spokesperson said all students and workers were accounted for, and said authorities believed the blaze might have started in the roof.

The cause of the fire is still under investigation.

More to come soon.