Seven children allegedly held and threatened the six-year-old girl before forcing her to undress and degrading her in different ways.
"It is so terrible I can hardly talk about it," the girl's father told the local Allehanda newspaper.
"I don't understand how such young people can be so evil," he added.
The girl was very upset after the incident. She received medical attention and was left with bruises, cuts and a split lip. According to her father she is in a "deep crisis".
The alleged assault was reported to the police as sexual molestation. The police referred the case to social services as the accused are under 15.
It is not clear which measures have been taken to support the girl since officials have refused to make any statements.
"I neither want to nor can I say anything," Lisbet Sander, head of individual and family care in the Härnösand municipality, told news agency TT.
"We are trying to find out what happened by talking to the pupils involved and with the parents," the school principal said.
The girl's father said that one of the reasons she feels bad is because a teacher at the school asked why she had not screamed when she was attacked.
"Now she feels guilty for not having screamed, despite the fact that the other children threatened her with violence," said the father.