What does it mean?
Why is "His parents will be speaking with his teacher" not accepted? In similar exercises the implied possession is accepted or even prefered.
That does indeed make a lot of sense, added now.
it should be 'his parents will be talking with a teacher'
Well, "to" is correct, but yes, "with" is closer to Polish, so it will be the main answer now.
Speaking with should be ok
Why the instrumental for teacher? Is he an instrument for his parents? And if yes, does this happen whenever someone does something with someone else?
Simply the preposition "z" (when it means "with") takes Instrumental.