While other kids were causing trouble, Xiao Ming behaved like a perfect choirboy.

choirboy 是由 choir（合唱团）和 boy（男孩）组成的复合词，字面意思是"合唱团的男孩"。在英语中常用来比喻"品行端正、举止规矩的男孩"，带有一种天真无邪、循规蹈矩的形象。