Most schools today are run by a School Improvement Team or a School Leadership Team which include parent representatives -- these teams help make decisions about the running of the school. Check to see if your school has such a team in place and ask to address the group to express your concern.
If neither of the above work or are possible, call the school system central office. All school systems have a chain of command, and the principal of your school has a supervisor. Ask to talk with him or her and explain what is going on at the school. If this still has no effect, keep going up the ladder -- talking with the school system superintendent and then if necessary, to members of the school board.
For information on children's self-esteem and how it might be affected by this, check the websites for the National Association for the Education of Young Children (www.naeyc.org), the National Association of School Psychologists (www.nasponline.org), or the Educational Resources Information Clearinghouse (www.askeric.org).