"Magistrum" is in the accusative (direct object) case, rather than being "magister" which is the nominative (subject) case.
Why is there no sound icon at the top of this page? In other courses you can find one there.
Is there anything wrong with Corinnae est magister ? (I.e., using dative of possession with the verb "to be.")
I've been adding Datives of possession for a lot of the sentences, I'll go add it to this one.
Added, it will take a few days for it to update perhaps. Magister Corinnae est was already set up by the way.