Police have detained two 15-year-olds, a boy and a girl, who attended the same school as the victim in Argenteuil on the outskirts of Paris, after her death late on Monday.
The killing has raised new concerns about juvenile crime in France after a spate of violence blamed on fighting between youth gangs.
Pupils interviewed by AFP outside the school in Argenteuil said the dispute started when photos of the victim “in underwear” circulated on the popular messaging service Snapchat.
“What happened to this young girl is utterly terrible and despicable,” government spokesman Gabriel Attal told reporters after a cabinet meeting chaired by President Emmanuel Macron.
“She was taken away by harassment, taken away by a vicious circle that contributed to this drama,” he added, vowing that those responsible would be punished.
Attal said: “We know that harassment begins at school, it starts in the classroom and the playground. But what is new in recent years is that it continues online after class.”
According to prosecutors, the mother of the detained 15-year-old told police her son said he and his friend had hit a young girl and she fell into the Seine, which flows through Argenteuil after leaving Paris.
A 15-year-old boy of Ukrainian origin was severely beaten in Paris in January, with the attack caught on video and going viral.
“We cannot accept a trivialisation of violence,” said Attal, adding that there needed to be a “penal response for minors that was quicker and more effective”.
Last week, Education Minister Jean-Michel Blanquer, Interior Minister Gérald Darmanin and Justice Minister Eric Dupond-Moretti chaired a meeting of justice and security officials to deal with the problem of teenage gangs, especially in the Paris region.