High school students in a wealthy Dallas enclave will find tighter security when they get to school after several threats were received over the last week.
   The intensified security Friday comes after a threatening note was found Thursday atop a hallway stairwell at Highland Park High School. It came one day after someone left a box of .22-caliber shells in a boys' bathroom. Another written threat was left in a bathroom Tuesday.
   In a letter to parents, Principal Walter Kelly says metal will be carrying handheld metal detector wands for searches if necessary. The letter tells parents and staff that school will be open regular hours.
   The school in January received three other hoax notes. A $7,500 reward is being offered for information leading to an arrest and conviction.