Columbine was far from the first school shooting. According to the Washington Post:
"The first recorded school shooting in the United States was in 1853 at a schoolhouse in Louisville, Kentucky. On November 2, 1853, Matt Ward shot and killed teacher William H.G. Butler with a pistol hidden in his coat pocket."
I think the very important point you’re missing is that schools did not exist in fear of school shootings before Columbine. There were no lockdown drills and crazy security measures for entering and leaving the building. So making a big loud noise would not make people instantly think someone was shooting up the school like it very well might today.
I know it's not the first, I never claimed it was. But as someone who is old enough to remember what life was like before Columbine, that was the one that changed everything. That's when we started having active shooter drills.