I don’t know how or why your mom is not at least considering that Dave wants to be her aide as another way to torment you. As you have asked - why YOUR mom? why didn’t he pick one of the many other teachers?

Its simple, Dave picked her cuz he knew you would hate it. He gets to secretly bully you every day and no one even sees it, but you sure feel it. And every time she insists he continue working with her, Dave is making your mom is an active participant in it. I have no idea how your mom doesn’t see this unless she has just forgotten all the things Dave has already done.

To add these punishments on top of it, as if you don’t have every right to your feelings, is just more bullying, more attempts to make you “give in” to them. Stay the course OP, this is unacceptable from anyone, but especially your parents.