you can report that as correct too. But when we talk about professions, we find a difference in Portuguese. In English, when you have "a/an" before an occupation, in Portuguese the normal way is omitting "um(a)". She is a nurse = ela é enfermeira. He'll be a journalist = ele será jornalista. But you can add "um(a)" if you have additional information: ela era UMA excelente professora.
how do you get she was "one" teacher form ela era professora? Wouldnt that be ela era uma professor?
Actually, is " Ele era UM professor" or "Ela era UMA professora" In articles we have masculine and feminine on it..