The biggest contributing factor was 9/11. My school had 3 pay phones for 1600 students. When 9/11 happened it was chaos. Tons of teachers were letting students use their cell phones (of they had one) so kids could call their parents, and no me had a problem with anyone that had a phone pulling it out of their bag to use it. It felt like almost overnight half the school had a new cell phone that no teacher cared about.
School shootings have just been a continuation of the fact that parents want to know their kids are okay at any given moment.