Aggression can take many forms - from very subtle, passive acts to violent outbursts. It often results when a student perceives a threat, feels frustrated and/or out of control. Some aggressive people express hostility immediately without regard for their circumstances or the people around them. Others deny their anger and frustration until their hostility builds to the point of an explosive outburst. Many times, persons who are verbally or physically aggressive feel inadequate and use hostile behavior as a way to build up their self-esteem. Often these individuals think that you will reject them so they become hostile and reject you first to protect themselves from being hurt. They may see you as attempting to control them and may lash out to try to gain some sense of control.
It is important to remember that the student is generally not angry at you personally, but rather you are the handy target of pent-up frustrations.
Overall, dealing with an aggressive student will be handled best by maintaining a firm, consistent and calm control in the situation (i.e., know what you are doing and what your goals are).