But a researcher is not surprised, and says the mutilation probably happened before the girls' families came to Sweden, and so is not illegal.
From last autumn Norrköping is a pilot municipality for Östergötland county council's work against so-called honour violence. This as part of a ten-year-old government order to the county.
Since March 2014 around 60 cases of FGM have been discovered in Norrköping, including a class where all 30 girls had suffered this mutilation. And 28 had been subjected to the worse form of abuse, where the clitoris and inner labia was cut away and the vulva sewn together to leave a tiny opening.
It was the school nurse who discovered that the girls had been injured in this way. The county council programme against "honour violence" involves extra education for school staff, so such abuse can be more easily discovered.
Since 1982 FGM has been illegal in Sweden. Since 1999 it is also punishable even of carried out abroad.
Local paper Norrköpings Tidningar reports that social services have now allocated extra resources to help the abused girls.
"We try to tell parents that they can be sent to prison if they come back from a trip abroad and the child has been subject to FGM," says Petra Blom Andersson at the centre for school pupils' health in Norrköping, to Norrköpings Tidningar.
But researcher Sara Johnsdotter says to news agency TT that she does not think the girls' parents have committed a crime, "If it happens abroad before the person has become a Swedish citizen or come to Sweden then it is not illegal." She says the important thing now is to see the girls are treated well.
"You don't have to have respect for the practice, but for the people who are affected by the practice," says Sara Johnsdotter, researcher in medical anthropology at Malmö University to news agency TT.
She says this is a non-story for her, and she would be surprised if all the girls were not subject to FGM, considering which country they come from.
The World Health Organisation says over 125 million girls and women alive today have been subjected to FGM in Africa and the Middle East.
No one knows to what extent girls in Sweden have been subjected to this form of abuse.