If you're a student in school and you see a gun sticking out of the pants of one of your fellow students, you tell a teacher and/or another school employee.
If you're a parent and one of your kids steals one or more of your guns, you call the cops.
If you're a student in school and one of your fellow students attempts to recruit you to help in a school shooting, you call the cops.
If you're a student in school and one of your fellow students hands you a suspicious package (they don't tell you what's inside), and you open it and find it has weapons inside, you tell your parents about it.
If you see someone threatening to carry out a school shooting on social media, you call the cops.
Here is an article with more information:
https://www.besafe.net/news/prevented-school-shootings-stories/