when the cat's away the mice will play

Meaning

1. people enjoy themselves more and behave with greater freedom when the person in charge of them is not there

From the same entry

• be the cat's whiskers
• let the cat out of the bag
• like a cat on hot bricks
• like a cat that's got the cream
• cat got your tongue
• look like the cat brought in
• not have a cat in hell's chance
• play cat and mouse with
• put the cat among the pigeons
• curiosity killed the cat
• like herding cats
• be raining cats and dogs