The bus driver, a 48-year-old mother who was seriously hurt in the collision, was “placed under judicial control and banned from driving,” said state prosecutor Xavier Tarabeux.
At least 20 others were injured when an express train smashed into the bus carrying children home from school in the southeastern village of Millas on December 14.
The driver told investigators that the safety barriers were open at the time of the crash.
But Tarabeux on Wednesday said “material findings” in the investigation suggested the barriers were closed.
The national rail network SNCF said that the barriers had been functioning normally, though witness accounts varied.
Both the bus and train driver were given toxicology tests that came back clean.
The crash was the worst involving a school bus in France for 30 years.