I am not surprised that the teacher hasn't had an incident of biting in that age group - it is very unusual for kids of this age to bite. Even more so, to bite hard enough to break the skin.
I would at a minimum schedule an appointment with your child's pediatrician. If for no other reason, to show the preschool that you are taking this very seriously. With this being your little guy's first introduction to preschool, it might simply be because of everything being so new and exciting. However, there is a possibility of an underlying issue and the best time to address it is before kindergarten. You do mention that he has had some anger issues that you have worked on and improved on, but you are working with him in a controlled environment - your home, and those skills may be difficult for him to transfer to a new environment.
When you meet with the school, letting them know that you have scheduled a peds apt will be a big help, but I would also ask them what kind of consequences are they giving for this behavior, are they always consistent even with different staff? Are they consistent with what you are doing at home (I realize he really isn't biting at home, but just generally for undesirable behaviors).
Good luck! I know it is super frustrating and scary that your child could be asked to leave, but hopefully between you, the pediatrician, and the school, some really good ideas will come out.