Learn Spanish (Latin America): Este mes, él está de maestro en la escuela. - This month, he is filling in as a teacher at the school.

While ser is normally used for professions (for example, él es maestro means he is a teacher by trade), using estar de plus a profession implies a temporary situation. Here, it means he is temporarily working as, or filling in as, a teacher just for this month.

Could I say trabaja de maestro instead of está de maestro?

Yes, trabaja de maestro is perfectly natural and means he works as a teacher. However, está de maestro places a slightly stronger emphasis on the temporary, acting nature of the job, which pairs perfectly with the time limit set by este mes.

Why isn't there an article before maestro, like está de un maestro?

In Spanish, you generally drop the indefinite article (un or una) after verbs like ser, trabajar de, or estar de when stating an unmodified profession. You would only use un if you added an adjective, such as está de un maestro excelente.

Does the structure estar de only apply to professions?