(A school district did not owe students a duty of care to prevent a teacher’s husband from entering the classroom and killing her in front of her students, thus causing the students emotional distress.  The husband’s violent criminal behavior was not reasonably foreseeable, as he had previously visited his wife at the school without incident.  Also, protecting against this sort of third party crime would require expensive and disruptive measures that would interfere with the school’s primary mission of teaching children.)