[You must be registered and logged in to see this link.]" target="_blank" rel="nofollow">https://en.wikipedia.org/wiki/List_of_school_shootings_in_the_United_States_(before_2000)
[You must be registered and logged in to see this link.]There were plenty of school shootings before Columbine, the number of them was increasing before Columbine (although that could also be a function of increased US population), and the media attention on them was also increasing before Columbine. The motives for school shootings also remained pretty consistent among shooters both pre- and post-Columbine.
So I don't think that the number of school shootings was really increased much or at all by Columbine. Sure, some shooters claimed to be inspired by Columbine, but that's sort of a chicken and egg problem. Were they inspired to shoot up their school by Columbine, or did they identify with Columbine because they already wanted to shoot up their school and it just so happens to be the most well-known school shooting?
The increase in the rate of school shootings tracks with two things -- deinstitutionalization, and the more widespread use of psychotropic medication in children and adolescents. I think those two things are more reliable contributing causes, but that's just my opinion.
As for specific shootings that would or would not have happened, well, that introduces the "butterfly effect" type of question, so I don't think we can say for sure with any of those.