Not "you shouldn't be in this class", but "you should should not be with me". It's kind of odd wording. And then "you're not doing what I'm doing". I almost wonder if she meant "you're not supposed to be doing what I'm doing". Like she was trying to convey that you were supposed to mirror her instead of following her exactly.
I had the same thought.
Me, too. I think it's probable that she was showing a more advanced variation. In many of the classes that I attend, the instructors will show the basic move and both a more advanced and more beginner variation. People who are still building a base should use the beginner move, even if the instructor models the advanced version for some of the reps in the set.
While yelling across the room like that wasn't the best move (a general "If this is your first time taking the class, you should use the beginner variation for all sets" or something similar would be okay), this could have been an issue of safety.
I really doubt that she was trying to tell you all to leave. I think it was just a reminder that you should be doing the beginner version rather than the advanced. If something similar ever happens in another class, ask for clarity right away. "I'm sorry, what should I be doing?" You could also approach the instructor after class to ask her to explain.