There weren’t many school shootings, and suddenly Columbine happened.
The thing is - Columbine wasn’t really a school shooting.
It was a failed bombing. The shooting was to get everyone into the cafeteria where they’d set up barrel bombs which luckily didn’t go off. In fact, the largest casualty attack in a US school remains a bombing from 1927.
As a school shooting, Columbine was also quite atypical, with two perpetrators.
But as soon as you now had what was really a failed bombing being covered by the news as a school shooting, suddenly thereafter were a ton of school shootings (that fit the normal archetype of a mass shooting with a lone perpetrator).
And each of those got a ton of coverage and the numbers of mass shootings went up yet again.
If you suddenly prohibited covering mass shootings in media (impossible because of the 1st amendment, but hypothetically), I am certain you’d see mass shootings drop by double digit numbers.
The fact that Columbine was so atypical of what events followed in its planning but was so close to what followed in how it was covered in the news tells a pretty damning story of the role of mass media in this phenomenon.
Also see:
Towers, S., Gomez-Lievano, A. Khan, M., et al. (2015). Contagion in Mass Killings and School Shootings. PLOS One. 10(7): e0117259. doi:10.1371/journal.pone.0117259
Lankford, A and Tomek, S. (2017). Mass Killings in the United States from 2006 to 2013: Social Contagion or Random Clusters. The American Association of Suicidology. doi: 10.1111/sltb.12366