“当老师”常见的英语表达有 become a teacher、be a teacher 或 work as a teacher，具体使用可根据语境选择：
1、 Become a teacher
强调“成为教师”这一身份转变或职业选择，例如：
She decided to become a teacher after graduation.（毕业后她决定当老师。）
2、 Be a teacher
描述当前职业状态，例如：
He has been a teacher for ten years.（他当老师已经十年了。）
3、 Work as a teacher
侧重于“以教师身份工作”的动作，例如：
She works as a teacher in a primary school.（她在一所小学当老师。）
其他相关表达：Teach as a profession（以教学为职业）
Pursue a career in teaching（从事教学工作）
根据语境选择最贴切的表述即可。