A 12-year-old schoolgirl threatened a teacher with a knife in an apparent murder attempt at a French school on Wednesday, prosecutors said.

No one was injured in the incident during English class in the northwestern city of Rennes but prosecutors said they have opened a criminal investigation into attempted murder.

The schoolgirl "came to class armed with a large knife with the apparent intention of killing her English teacher," Rennes prosecutor Philippe Astruc said.

"During the lesson, in class, she brandished the knife at the victim who fled running," before the girl was disarmed by staff of the Hautes Ourmes junior high school, he added.

"The pupils, shocked, were immediately moved to safety," the local education authority said.

The girl underwent psychiatric examinations in hospital which concluded that "the minor was 'dangerous for herself' and that her condition required care in a specialized environment," Astruc said in a statement Wednesday.

A young girl from the school told Agence France-Presse on condition of anonymity that there had been a dispute between the schoolgirl and the teacher who allegedly confiscated her cell phone last Friday.

At a press conference, Astruc showed drawings of a kitchen knife, which he said was 17 centimeters long.