Maybe tell her its annoying and if she still teases you ignore her
You tell him he is a jerk, and to leave the girl alone. Tell him she is your girlfriend.
You should try not to let it bother you, because if people know it doesn't bother you they'll stop. if that doesnt work tell a trust adult.
no, maybe he's just trying to get over you. Then later, he'll ask to be just friends. Don't worry, though. He'll come when he's ready.