Three female French students in their early teens were threatened with a large butcher's knife as they walked through an Oxford estate.
One girl was grabbed from behind and had the knife held to her throat by one of two attackers who struck in Blackbird Leys on Saturday night.
She cut her hands trying to get the knife away from her throat. Some of her hair was also cut off.
Police officers are trying to establish what happened through interpreters.
The other two girls were not injured but a purse containing a small amount of cash was stolen.
The robbery happened in Blackbird Leys Road, near a park.
The girls are staying with host families in the area and are attending the EF Language School in Oxford.
The offenders are described as white youths, aged about 16 or 17.