According to the local media reports, most of the victims were reported to be aged between 13 and 17 years and death toll may rise as nine children are in critical condition.
The injured were taken to the hospital in helicopters.
The bus was carrying the students from the Christian Bourquin secondary school at Millas to their homes in two villages, Saint Feliu d'Amont and Saint Feliu d'Avall.
"I don't know what happened, but the school bus had been actually cut in two by the passing train," Olive told the local radio France Bleu Roussillon. "The bus appeared to have exploded. People were being very dignified but everyone was shocked," he said.
French Prime Minister Édouard Philippe, who met the victims' families, described the accident as a "terrible drama".
"The circumstances have not yet been determined and will be the subject of a legal inquiry to throw light on the circumstances of the drama," he said.
The French Prime Minister said 24 people had been caught up in the collision.
However, he added that the severity of the crash had made it difficult to identify some of the victims.
"The priority at this stage is to give precise information to the families who are living through a period of anguish that we must make as short as possible. The communication of this precise information relates to the identification of those who have died or been injured. This process has begun, but it is extremely difficult," he said. (ANI)