长大以后当老师可以用 "become a teacher" 来表示。这个说法来源于英文 "to become" 的意思是 "变成"，"a teacher" 则表示 "一名老师"。下面是一些例句：
I want to become a teacher when I grow up. 我长大后想当一名老师。
She became a teacher after she graduated from college. 她大学毕业后成为了一名老师。
He has always dreamed of becoming a teacher. 他一直梦想着成为一名老师。