Police in Hillsboro say a 15-year-old high school student has been hospitalized for treatment of a head injury after an alleged assault.  Witnesses say a 17-year-old student threw the victim to the floor Friday in a hallway between classes.  No names have been released.  W-M-T-V reports surveillance video shows the older student walked up behind the victim, punched him in the head, then grabbed him, picked him up, and threw him down on the concrete floor.  The 17-year-old student was arrested and charged with felony assault.