Why is it magistra and not magister?
Because "mea" gives away that we are talking about a female teacher. If it was magister, it would be "meus".
But it asks us to translate it. There are other questions asking us to translate English into Latin that don't come with word banks.
Why not "qui est magistrum mihi"?
It should, I think, be "Quis est magister meus", which is accepted
The question that preceded this one was «Write "teacher" in Latin»… and it did not accept anything but "magister" as an answer. The report button only gave an option to say "The pictures do not match the given word", and nothing else :(