Portland police Senior Constable Leah Sutherland said unknown offenders broke into the school on the corner of Portland's Edgar and Fawthrop Streets about 11.15pm on Sunday.

She said offenders broke the window of a door in order to gain entry to an empty portable.

"They've looked around but as the school is empty they obviously didn't find anything to steal," she said.

"They have triggered an alarm and then fled the scene."

Senior Constable Sutherland said it is believed the damage to the portable door was upwards of $500.

She said police wished to speak with anyone who heard something or a saw a vehicle leaving the area at the time of the offence.

Anyone with information should contact Portland police on 5522 1500 or Crime Stoppers on 1800 333 000.

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