The mother called the teacher and angrily berated her, calling her a poor teacher for not searching out the persons to whom the unmarked work belonged. This was a poor communication process at best. She completely missed the point that children have to learn to accept the consequences for repeatedly not following directions. Children have to learn to be responsible. One way to ensure that your children will still be living with you at age 35 is to prevent them from learning that lack of responsible behavior results in undesired consequences.
Nobody wants to hire an adult who has never learned to be at work on time or complete work properly.
This mother failed to recognize that her child's teacher was doing what a responsible teacher should do. What the parent should have done was communicate. Find out why the child got a zero. Seldom do we get by in life with being ignorant about other's expectations of us. You have to define your role in life and live up to it.