Good luck Crisc,
I wouldn't be too worried about the teacher's reaction for a couple of reasons.
First of all, it sounds like the school is already taking action in the form or additional assessments, hopefully in good faith to make sure he is going to be accomodated.
Also, remember, this is the teacher's job. This meeting is called because something is basically going wrong, not right. It's basically like being called into your bosses office; and not to be given a promotion, KWIM?
Many kids do act TOTALLY different when their parent is in the classroom and it is sometimes disruptive to the whole class dynamic. My DD6 totally regresses and starts talking like a baby when I'm in the class. It's awful. ((shrug)).
On the other hand DD8 is fine when I'm at school.
Lastly, she probably is a little freaked out that she didn't have time to "plan" something for your visit in terms of putting her best foot forward. Most teachers would do that, so she's probably feeling a little vulnerable. It would work in your favor if you are sensitive to this. However, it is much better for YOU to view the class without her making any preperation.
Hope you have a great meeting a fun weekend.