Have you asked the parents if they do play fighting or play biting at home? Sometimes parents pretend bite their kids (with no teeth of course) as a form of play and then the kids think it is playing. I had a little guy who started doing this and the dad admitted that he play bites. Dad stopped and the problem disappeared. Plus, I shadowed him for a week or two and interuppted him before he could land the bite...he got out of it really fast. Some do not stop so easily though I feel your pain! Biting is the worst!