Helsinki Police apprehended the woman last Thursday and formally arrested her on Sunday.
In a press release issued Tuesday, Helsinki police detective superintendent Jonna Turunen said that the woman had been plotting an attack for some time but police managed to thwart it.
"The preliminary investigation has started just now, but this is a case of [someone] preparing mass murder at a school," Turunen said.
According to Yle's Helsinki regional desk, the woman had been preparing an attack since March of last year and continued until the middle of this month.
Police said they have informed the school where the attack was being planned to be carried out, but did not specify which school it was.
"We have discussed safety issues with the school principal," Turunen said.
Police said they would release further details about the investigation at a later date.
The woman will be indicted by November 1 at the latest, police said.
According to newspaper Helsingin Sanomat the woman resides in the Helsinki district of Pihlajamäki.