The teenager was arrested at his high school in Stenungsund on Tuesday.
Police have classified the suspected offence as child rape but have otherwise released few details concerning the case.
“We have seized a number of items and interviews have been conducted with the girls,” said police spokesperson Ulla Brehm to the GT.se news-site on Wednesday.
Brehm added that the girls, reported to be ten-years-old, were “feeling well under the circumstances” and that they did not sustain any physical injury.
The alleged attack took place on Monday afternoon after the school had closed for the day.
The school reacted to the news by deploying extra staff resources on Tuesday, with the school health authorities involved together with a psychologist.
According to local news-site, nyhetersto.se, the school playground bore little resemblance to a crime scene on Tuesday.
“We are having a regular school day, school is not cordoned off and there is calm. We try to be there for all children if they have questions,” Agneta Ströberg at the school said.
According to nyhetersto.se the 18-year-old was collected in his classroom by uniformed police later on Tuesday.
Following an interrogation he was notified of the suspicions of child rape.
Items found at the alleged crime scene have been seized and sent for forensic examination.