when the cat's away, the mice will play

When someone in authority is not present, those subordinate to that authority do whatever they want. A: "If I leave my classroom for even a moment, I come back to find the place in chaos." B: "When the cat's away, the mice will play."

when the cat's away (the mice will play)

When the authorities are absent, people will break rules and do as they please. This proverb, which exists in numerous languages, appeared in several different forms in English in the seventeenth century. Thomas Heywood used it in A Woman Kill’d with Kindness (1607): “There’s an old proverb—when the cat’s away, the mouse may play.” Today it is often shortened.