A Bay of Plenty School is open for "instruction as normal" following a suspicious fire.

Fire and Emergency NZ were called to Allandale School in Whakatāne about 6.40pm on Wednesday.

"The small prefab classroom was well involved in fire," says Fire and Emergency New Zealand shift manager Paul Radden.

There were two fire trucks from Whakatāne there at the scene for about 90 minutes.

No-one was in the classroom at the time.

A fire investigator will determine the cause of the blaze.

"A huge "thank you" to our vigilant neighbours and to the amazing women and men of the Whakatāne Volunteer Fire Department," says Allandale School In a post on its Facebook page.

"Without their quick response, damage to our school could have been a lot more severe. Ngā mihi nui ki a koutou!

"Sadly, Fire Investigators are treating the event as suspicious. Anyone with any information about the fire are asked to please contact the police on 308-5255."

Additional reporting by Stuff.co.nz