At a younger age, children may not fully understand the consequences of their behavior and how they treat others. Some kids may tease others and not realize that the victim does not take it as a joke. This, in turn, can lower a kids self-esteem.
Also, if the teacher is not giving the child enough credit for doing good things, then the child may not feel as inclined to do so. Some teachers even yell and name call at little kids! This especially can impact a child's self-esteem. Kids look at adults as the lead figures in their lives. They can make or break how a child feels sometimes.