The 14-year-old victim from the Paris suburb of Argenteuil, named only as Alisha, was found in the Seine river in March 2021.
Prosecutors say the two suspected killers, a boy and a girl aged 15 at the time who cannot be named as they are underage, were her former friends who had been expelled from school for bullying her online and in person.
After Alisha met the girl near a motorway bridge over the river, they were joined by the boy who “hit her by surprise in the face, yanked her hair and slapped her so hard she fell to the ground,” the investigators found.
The two then dumped her into the river “to get rid of the traces of the violence they had committed”.
They then returned to the boy’s home, changed clothes – his were covered in blood – and went out to eat in central Paris.
But they had told the whole story to the boy’s mother, who herself called the police.
The youth court, holding its hearings behind closed doors until Thursday, could sentence the pair to up to 20 years in jail for the murder.
As well as physical and verbal abuse, the suspects are accused of harassing Alisha over pictures of her in underwear that were stolen from her phone and posted on social network Snapchat days before.
Prosecutors said she was the victim of a form of “revenge porn” – in which exes share intimate images of their former partners online to cause emotional harm.
The killing prompted an outpouring of support for Alisha’s family and indignation over cyber-bullying from across France.
More than 2,000 people marched in her memory days after the killing was discovered, with her mother saying that Alisha was “very serious at school” and dreamed of becoming an accountant.
Her parents and one of her siblings were at the trial with a large portrait of Alisha on Monday, an AFP journalist reported.