By SARA VANDEN BERGE   Parents of students at Morgan Mill ISD say they are concerned about a student who allegedly threatened to bring a “sniper rifle” to school this week.   The student – a second grader – was reportedly on the school bus when the threat was made on Tuesday afternoon.     On Thursday, parents received a message from Morgan Mill ISD Superintendent Wendy Sanders, which states:   “Dear Parents and Guardians,   “This message is to inform you of a safety-related