Five children travelling on their school bus when an express train struck it last Thursday at Millas near Perpignan remained in a serious condition, according to the prefectural office.
Another 12 were hurt, including the coach driver, in what was the worst crash involving a school bus in 30 years in France.
"The bus was very badly hit," the public prosecutor in Perpignan, Jean-Jacques Fagny, said
France Bleu regional radio reported that the train ploughed into the rear of the bus. Pictures posted on French media showed the bus had been severed in two.
Around 95 emergency workers, backed by four helicopters, were deployed as part of the rescue effort. The crash site was cordoned off to the media.
President Emmanuel Macron had tweeted saying "All my thoughts for the victims of this terrible accident involving a school bus, as well as their families. The state is fully mobilised to help them."
Investigators are probing the cause of the collision to determine if a safety barrier malfunction at a level crossing was the cause.
The bus driver, a 48-year-old woman, says the crossing barriers were open at the time of the collision, according to her employer, while France's SNCF national rail operator insisted Saturday the barriers had been "functioning normally".