Depression has been part of the human condition for thousands of years. Guns have been around for centuries. School shootings are a recent phenomenon.
What changed in the past 10-15 years?
It's not as recent as you might suspect. When I was in 5th grade about 26 years ago, a 16 year old girl opened fire on Cleveland Elementary in San Diego killing 2 people and wounding others. I remember it quite clearly because the principal from my school took over for the principal that was killed. There was a song written about the incident called "I Hate Mondays", which is the reason she gave the police that day.
God out of the schools. God out of the family. It's all downhill from there.