The student did NOT know the killer. A threat was sent to students online, and some students took it seriously.
*** SNIP ***
>>> “Many Oxford High School students appear to have had concerns for their safety before the shooting. One student, who spoke to the media after the shooting, says that he and several friends stayed at home on the day of the incident due to threats that had been circulating online. He said there was an eerie feeling amongst his fellow students. The boy’s mother reportedly told the media that an explicit threat was issued to students online that read: ‘If you come to school today, you will get shot.’”
One cannot blame the school if the kids seeing the threats only told each other and not those in authority. The kids just have more eyes and ears.