This all sounds very stressful! Where are you hearing all this information? From DS, or the scribe, or other kids/parents/teachers?
I know you're working but can you take a couple of afternoons off to sit in on this class? If it were me I'd take turns with DH to turn up randomly to the class during the year just to let her know you're onto her and you're watching her. (Sit menacingly in the back, pointing from your eyes to her … ) I like to see for myself whatever DD complains about - and it always turns out she's right.
I'm not sure either whether your DS should have to put up with all this just because you're determined not to let her "win" and drive him away
Good luck with the meeting