This is the 4905th most frequent French word.
sage
Wise; well-behaved.
Here, 'sage' is an adjective meaning 'well-behaved' or 'calm', referring to the student's conduct during the class.
Le professeur a remarqué que l'élève était sage pendant tout le cours.
The teacher noticed that the student was well-behaved throughout the class.
In this sentence, 'sage' again means 'well-behaved', used in a simile to convey calmness and quiet behavior.
Émilie disait que sa petite sœur était aussi sage qu'une image.
Émilie said that her little sister was as well-behaved as an image.
The word 'sage' is used as an adjective meaning 'wise', describing the old man's characteristic to give good advice.