The question "what is the problem?" is often more complex than it seems. Of course, you need to know specific ways in which your kindergartner is acting out. But you also want to ask when such behavior occurs, how often and under what circumstances. Ask if certain things tend to bring out the behavior more than others, such as a child acting cranky because he needs a nap.
This is an especially crucial question to put to the teacher. Try to correlate the behavior with events that happened recently in your household. For instance, if the family moved to a new residence in October, and the behavior began around that time, the child's acting out may be related to the move. Or, if a family pet died several weeks ago, and the behavior made itself known at that time, the child's grief or anger may be the cause for problem behaviors.
This is where you hash out a plan with the teacher about how to improve your child's behavior. You can take a variety of steps at home to improve a child's performance in school. For instance, if your child is constantly fatigued and falls asleep in class, you can make sure that she goes to bed a little earlier each night. Providing a nutritious breakfast in the morning also energizes little tykes for a day of learning and play.
Although it's not good to get caught up in worst-case scenarios, it is wise to determine what will happen if the behavior does not improve. Your child may need to be referred the child-study team, a private psychologist or a special education program. It is possible that he may need to go to a special school, or that he may even be expelled from the present one. But these are usually the last options chosen for behavior-challenged kindergartners---there is much that can be done before it gets to that point.