મહેતાજી ગયા એટલે નિશાળીઆને મજા.

mahētājī gayā ēṭalē niśāḷīānē majā.

Literally: When the schoolmaster leaves, the schoolchildren have fun.
What it means: People take advantage of the absence of an authority figure to relax or misbehave.
The English proverb Shah set beside it: When cats are away, mice are at play.

Also said as

ગુજરાતી કહેવતસંગ્રહ (Gujarati Kahevat Sangrah) (1921) · read from the scan by machine — the Gujarati itself is unchecked · English is machine-made and nobody has checked it yet

Heard it differently, or know we've got the sense wrong? Tell us — that is exactly the kind of correction this needs.