I may have this context wrong, but as in 'their', I am implying a gender neutral term for the teacher. Do you just automatically place 'his' or 'her' in the sentence and assume the teacher's gender even if you are not sure?
That should be fine. The gender is rarely assumed, but in this context you would probably know the gender of the teacher, so there wouldn't really be a reason to use a gender neutral term. Often the sentence would be rewritten to exclude any references to gender, as there aren't really any gender neutral pronouns in Norwegian.