My oldest DD started biting at that age too (maybe just a tad younger, I don't remember exactly) and I was just as baffled as you are. Oddly enough, she only bit her cousin, who is about 16mos older than she is. This all started happening while my brother, SIL, and nephew were visiting us from out-of-state and staying with us. We just couldn't keep them that far apart in our little apartment. Turns out, my nephew was doing little subtle things to her to provoke her. If she was sitting on the floor coloring, he would walk by and deliberately and carefully step on her hand. That did NOT make it okay for her to bite him, but it really helped in resolving the issue that there was a reason that she thought she had to resort to biting, etc. The teachers really need to see what is provoking your DS, that's their job. If they can't keep track of all the kids, they need more teachers or fewer students. Your DS obviously feels the need to defend himself, or something is upsetting him, and they need to find out why.