"Teachers in almost all states must have a Bachelor's degree with the appropriate teacher preparation course and complete either a content-based or teaching-based Master's degree within a stated number of years. Additionally, to be permanently certified, many states require that teachers pass exams on pedagogy, general knowledge and knowledge of a content area. Some states require teacher candidates to be fingerprinted prior to certification." Wikipedia.

I'm guessing, but I think you would not need TESOL certification, but would need to do what is required to get a licence. The Wiki article explains further about standardised tests.