Are you sure Corinna is a "magister" is possible? and not only a "magistra".
If Latin is like Romance languages, when a feminine does exist, and when the noun doesn't have a neutral meaning (like "enfant", "professeur", etc, in French), we must use the feminine.
Corinne est un maître. Would sound weird. "maîtresse" would be used.
But "professeur" is a neutral noun in French (but now it has been feminized in very modern French)
Corinne est un professeur, would still work, nevertheless.
Corinnae est magister = C. has a teacher (There is a teacher for C., who's in the dative case in this sentence).
Corinna est magistra, yes, certainly! If she's a teacher, she is a female teacher. (In this sentence, she's nomin., subject of est, and described by the nominative/pred. noun, teacher.)
The ending -em goes on nouns (like pater: patrem, mater: matrem, frater: fratrem) that belong to the 3rd declension. But magister (male teacher) belongs to the 2nd declension, as you can see from the ending magistrum (the word for boy, puer: puerum, follows the same pattern). The female teacher (magistra: magistram) belongs to the 1st declension.