Authorities gave no clue as to the motive for the shooting, along a highway near Drôme in the south-east.
Initial investigations indicated the shot was fired from a hunting rifle.
Three dead in suspected gang shooting in France: source
Prosecutor Alex Perrin said the injured were hit by shattered glass.
The bus was carrying 75 tourists including children on a school trip.
"Five people suffered light injuries, while the sixth was more serious," Perrin told BFM TV.
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