I hesitate to post this as I don’t want to exacerbate OP’s fears, and it’s hard to know full stats because things are counted differently in different states, but death by gunshot is actually the second leading cause of death for young people in America. It is not all school shootings, but *some* level of concern here is not inconsistent with the facts.
https://www.cnn.com/2018/12/20/health/child-gun-deaths-rising/index.html
OP, if it is to the point where it is disturbing your sleep I suggest therapy. The usual advice others have shared may not be enough. It’s hard not to worry in general but the media coverage and lack of action on this one make it extra hard. Perhaps volunteering on the issue will give you some comfort, a sense of control and action?