An early wake up call for a Christchurch school principal this morning.

A fire at Linwood College's Aldwins Campus has caused extensive damage to a two-storey classroom block just before 3 o'clock this morning.

Principal, Richard Edmundson, says the school is currently based in Avonside, while the Aldwins Campus awaits demolition and a $44 million rebuild.

"They are empty buildings, they are due to be demolished, no kid has lost their work or teachers their resources."

However, he says there's still a sense of loss contemplating all the teachers and students who've been through over the past 60 years.

"It's very sad to see smouldering framework in the early morning, and then later on this morning as well. Not nice."

Police and fire are investigating.