Rather than thinking of a teacher necessarily, one can think of an individual acts more like a mentor or coach in certain settings.

不必要一定要有个老师，你可以想成一个人在某个领域,作为指导者或是教练。

麻省理工公开课 - 媒体、教育、市场课程节选

The reason I got to be a soccer team captain in the first place was because my coach said,

我最初之所以成为球队队长是因为我的教练说，

足球部队长的领导力 - SpeakingMax英语口语达人