NOT AGAIN!!!!

As I picked Seth up from daycare yesterday, his teacher said, "Seth has a few accident reports for you to sign." One was that he was behind a door and a teacher didn't see him and when she opened it, she knocked him in the head. He's okay and only has a small bruise. The second one was that he accidentally fell on top of another child and became frustrated when he could not get up and bit the little girl. The teacher said it wasn't hard, but he has also been trying to bite me some when he gets frustrated. Anyone have any ideas how to stop this??? I know it's probably a phase and his pediatrician said that most kids bite around this age, but I still don't like it.