Is a female teacher also a masculine Der Lehrer? Wouldn't she be a "Die" since she is a "sie" ?!?
In German, a female teacher would be called "die Lehrerin."
Many other words for professions or occupations add the "-in" suffix to indicate that the job is being done my a female.
Der Arzt (male doctor) -> Die Ärztin (female doctor)
Der Student (male student) -> Die Studentin (female student)
hope that helps