A behavior therapist would tell you that yes, rewarding undesirable behavior will definitely get you more of that behavior behavior. So letting her get out of the class is a problem, since she's using the bad behavior to get what she wants.
You need to set it up so that being there is more rewarding for her than not being there, and so that "breaks" are on the teacher's schedule, not hers. Find her currency and use it. It's OK to schedule in extra breaks for a kid who needs it, but certainly not if she demands the break by threatening people.
Does she have an IEP going into the public kindergarten? Setting up a situation like this for success usually takes extra manpower (e.g. a special ed teacher or aide who's there to dole out appropriate prompts, rewards, and consequences so that the classroom teacher isn't tied up dealing only with one kid's behavior).
HTH,
DeeDee
