The children were returning home from a school trip in the Alps (Picture: Shutterstock)

A school bus carrying 40 young children home from a trip to the French Alps has plunged down a wooded slope and into a creek, leaving the driver and his partner hospitalized with serious injuries, authorities said.

Eighteen children and one other adult were treated for minor injuries, said Fabien Mulyk, mayor of the town of Corps, where the incident happened. 

He said the children were ‘all doing well.’

The regional prosecutor has opened an investigation into the incident. The mayor said the ‘most probable scenario’ is that the driver had some kind of medical problem while behind the wheel.

Local officials told French media that weather conditions were clear when the bus left the road, and no ice or snow on the section where the bus vehicle crashed. They said all the children were wearing seat belts.

A photo from the town published online showed the bus leaning off the slope of a ravine in a tangle of dried branches with its windshield shattered.

Eighteen children and one adult have been treated for injuries.