The teacher had the children stand on one leg to practice balance.
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history
The earliest clear written source is Qing novelist Li Ruzhen's 《镜花缘》, chapter 74: 我是“金鸡独立”，要一足微长 ('I'll do Golden Rooster Standing on One Leg - one foot needs to be slightly raised'). The phrase became fixed as both a tai chi posture (太极拳金鸡独立势) and a general description of one-legged standing.
culture
In tai chi (太极拳) and xingyi (形意拳), 金鸡独立 is a named form where the practitioner stands on one leg with the other knee raised and a hand extended - used for balance training and as a combat stance.
