Have you spent any time in her class with her? if not, you need to! I wouldn't even set up an appt, i'd just drop in one morning (and if they don't let you do this, i'd make sure i thought twice about sending her back) dropping in takes away any time needed to prepare for a visitor (they shouldn't need to prepare). See how the teachers are talking to the children. Are they speaking this way to the children or do they respect the children? Are the children being allowed to speak this way to one another? If so, something needs to change nowbefore it has any lasting effect on her.
Alternately, did you ask her teachers if there may be a child in the class who has a mouth like this--maybe she's picking it up from another child.
also, do they feed her snack there? Could she be eating something that doesn't agree with her, maybe an ingredient that her body may be sensitive to?
either way, it sounds like, if it's not just exhaustion or hunger, someting is going on there.