You clearly don't have kids. You drop them at school and for maybe 8 hours a day, as a parent you have 0 control over whats happening. Bullying, peer pressure etc. How is that also parent's responsibility?
Parents shouldn't buy smart phone to kids, period. There are 0 reasons kid of say 15 or lower should have one instead of dumb phone (or new commodore one with google maps and whatsapp but no browser or apps), in same way we don't allow cigarettes or alcohol to be given or sold to them. Helicopter control freak parents can go to hell, their mental issues shouldn't be messing their kid's lives. But to claim parents have 100% responsibility could work only if kids spend 100% of the time with them.
If you don't have any influence over how your kid acts at school and what he learns from the experience there then you're not parenting.
At some point your kid is going to move out or you will die and then you will have even less influence. School is a test for that.