Plz don't take this the wrong way, but biting is a normal and even expected stage in children's development. It, literally, happens to every child, in some more than others. It can be frustrating, especially if your child was hurt, but teachers can't keep their eyes on one particular child all day long, accidents happen. Of course, dealing with this issue is another story. Teacher should've told you about this right away, in details, show the "ouch" report, and assure that they will take measures to prevent this from happening again.