How do you say she is the teacher in spanish?

If you are talking to a teacher, you are most likely going to be using the Usted form of conjugation because they are going to be older or of higher level then you.

You can either say:

Usted es una maestra. (To a female teacher)

Usted es un maestro. (To a male teacher)

If you want to use the Tu form of conjugation then you would say:

Tú eres un(a) maestro/a.