Given the number and type of huge life-changes this child has had to face recently, no wonder she is not interested in behaving well or learning. In between the divorce and the death, she's seen her entire world fall apart. She needs help in sorting out feelings that could include anger, fear, abandonment, frustration at her powerlessness in the face of the changes, and inability to focus on academics due to all the other things going on in her life.
She is going to need counseling to help her work through the multitude of emotional concerns. From a behavioral standpoint, a daily recordkeeping system could be instituted to track amount of time behaving appropriately with rewards given for specific periods of time accomplishing specific goals.
For example, a chart on a corner of her desk could allow for a tally every time she hits or kicks a child. If she keeps the chart clean for a given period of time (start short - maybe all morning or all afternoon?), she may have the last ten minutes of the day taking part in some activity that she has chosen as a reward (free play corner, being read to by a caring volunteer, computer time?).
I would also explore the possibility of instituting some bibliotherapy, perhaps using a parapro or volunteer to share in reading very easy books about the death of a grandparent with her, followed by very gentle and sensitive opportunities to talk, draw, make music - whatever will help this child begin to find new ways of remembering and honoring her grandmother's memory.