Have you thought the teacher is talking in the 3rd person rather than non binary?
It's not common/standard English to refer to a single person as 'they' in the third person when you know the actual person and his or her sex that you're talking about.
I'm emailing regarding Jessica, and the issues with them not completing their homework. This has happened a lot and I've spoken to them on numerous occasions.
Yes, my response to that would be to ask why Jessica is being singled out or has been made the spokeswoman/scapegoat when the whole class (or multiple members of the class) have reportedly not been completing their homework.
Heard your grandma's been poorly. How are they doing?
If you said that to me, I'd naturally assume you were referring to the medical team looking after her and say something like "Well, they've run some tests on her and are continuing to monitor her situation before they can decide when she'll be able to be discharged." Of course, 'they' could indeed just be referring to a single doctor or nurse, if you don't know their name and what sex they are.
If you said 'she' or 'her', it would be obvious that you meant Grandma. Language is designed/has developed as a sophisticated way of communication - why go out of your way to complicate matters and confuse people?
but the persons gender choice has already been stated by use of the word "grandma"
You're allowed more than one sex signifier! Just because you've already identified her as a female by calling her grandma, doesn't mean that you don't need to use female pronouns in continuing referring to her - the opposite, in fact.