when the cat is away, the mice will play 山中无老虎,猴子称霸王；阎王不在；小鬼跳梁

【例句】
Joe seems to be on his high horse these days. This is "When the cat's away, the mice will play" .
乔这几天很不可一世，正应了那句“山中无老虎，猴子称霸王”。

As soon as the teacher left, the students started to make a fuss. When the cat is away, the mice will play.
老师一离开，学生们就闹腾起来，真是山中无老虎，猴子称霸王。