I agree with a lot of your points; however, bulling was actually much worse in previous generations than it is now. Previous generations also had far more access to firearms than kids today do as well.
I think a lot of it is that we as a society have built up these kids life expectations as small children by telling them they're special, equal, important etc. It's the end result of the whole participation trophy thing in many ways.
"I'm just as important and special as Johnny, so why is Johnny crushing puss after school now while I play World of Warcraft by myself? Why does Johnny get to go to parties that I'm never invited to. Why does johnny and other dudes just like him make fun of me constantly. This isn't the way it was supposed to be........."
I think this combined with psyche meds used to treat an increase in mental illness due to a ton of shit like poor diets, more stress, and an unbelievable amount of violent entertainment have contributed greatly.
Entertainment is definitely a big one. While kids had more access to guns in the 60's and 70's, it wasn't really until the 1980's that movies like Robocop, Scarface, Red Dawn etc started to become standard entertainment. The kids that grew up watching these movies were the first wave of school shooters like Kip Kinkler, Eric Harris, and Dylan Klebold.