TV images showed a long line of ambulance and emergency service vehicles near the crossing where the collision occurred.
Prime Minister Edouard Philippe described the crash as a “terrible accident” and said on Twitter he was heading to the scene, more than 850 km from Paris.
“All emergency services have been mobilized and a crisis coordination unit set up,” an official at the local Millas townhall told Reuters.
The interior ministry said seven people in the bus, which was carrying teenagers aged between 13 to 17, were seriously injured among the 19 aboard.
An SNCF spokeswoman said the train was running at 80 kilometres an hour at the time of the accident and 25 people were on board. Three of those were slightly injured.
France has suffered several train incidents in the past few decades. In 2013, a train derailment in central France killed at least six people. One of the deadliest was in 1988, when a commuter train heading into Paris’ Gare de Lyon crashed into a stationary train, killing 56 people, after its brakes failed.
— Reporting by Emmanuel Jarry, Caroline Paillez and Arthur Connan; Writing by Richard Lough and John Irish; editing by Mark Heinrich.