Pupils who arrived to the Oulunkylä primary school in Helsinki were met with a big surprise on Monday morning.
About a third of one of the school's classroom ceiling had collapsed, covering desks and chairs with piles of insulation and construction materials.
No one was injured in the incident, as the ceiling had apparently come down over the weekend, and school officials still do not know why it happened.
The city's urban environment division said it suspects the incident may be related to errors made in renovation work on the building which was completed about two years ago.
The school's rector Outi Rinta-Filppula said an entire wing of the school building - a total of 12 classrooms - have been closed off so that they can be inspected.
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"They will be temporarily closed in order to investigate whether there are any issues with the structures in other classrooms. In the interest of safety, 12 classrooms have been closed, so that nothing unpleasant occurs," she said.
Makeshift classrooms have been organised in the school's gym and music rooms, and some classes have joined groups in other rooms, according to Rinta-Filppula.
The affected classrooms will not be in use for 1-3 weeks, according to the school.