R.A.
Obviously this teacher reminds him of some person in his past who had a negative impact on him, even traumatized him in some way. I would talk with the principal about changing teachers. This is not normal behavior, nor is it appropriate for him to have this obsession with her.Typical adolescent boys always seem to poke fun or make fun of people in some way, but not to this degree, and not obsessively.
I am concerned. Especially with the negativity. I would discuss all of this with his therapists. Let them advise you on how to proceed. I would call them today and ask to speak with them about this situation.
Added: I figured that she looked quite grandmotherly. He needs grounding, and constructive discipline. No adolescent likes being told what to do, however, most will listen and follow rules. His negativity towards her, is something that needs to be explored further. I am sure you tell your son what to do, and he sees you as an authority figure as well, but he is not obsessed with you or hate you . It's a different issue that he has altogether. I bet their is a lot of hidden issues and layers that will need to be worked on in order for him to make progress.
What does the teacher say about his attendance and behavior in her classroom?