The driver had told investigators neither the barrier nor the warning lights were operating at the time of the collision at the level crossing in the southern village of Millas.
She was charged with involuntary homicide within days of the December 14th crash, France's worst accident involving a school bus for three decades.
"Everything was working perfectly," said Jehanne Collard, a lawyer for some of the victims' families, referring to an accident report from state rail operator SNCF, which has not been made public.
"One could certainly imagine that SNCF rigs its reports, but I have trouble believing that," she said.
"The SNCF knows that there is going to be a judicial probe and that it would be an enormous error (to lie)."
The French rail operator said the functioning of the crossing's safety features is "under the jurisdiction of the judicial investigation, with which SNCF is cooperating fully".
Collard, speaking on French news channel BFM, said the train involved in the collision was running nine minutes late at the time of the crash.
"The bus driver may have been surprised to see the barrier down, which could maybe explain why the accident happened," she added.