At lunchtime on Friday, the rescue service was alerted to the high school at Teleborg in Växjö after fires in three toilets.
The fires started every few hours, but did not spread and could be extinguished by staff on site.
In the afternoon, two students could be arrested on suspicion of having started the fires.
- From the witness observations we have, we suspect that we believe has started the fire on two of the occasions.
There are two girls under the age of fifteen, says Samir Cehic, on-duty preliminary investigation leader at Växjö Police.
Danger to life
The criminal classification is aggravated arson - partly because there was a risk of spreading, partly because the fires were started at a school where there was a danger to life if the fire had spread.
The two girls were formally arrested, but the arrest was later lifted by prosecutors, as they are not criminals.
On Friday afternoon, however, they were questioned in the company of their parents and lawyers.
As this is a serious crime, the police will continue the investigation, which will then be handed over to social authorities.