My brain decided to dig up some memories from when I was in 5th grade for some reason.
While I've never studied the case in depth I do know what most people know or maybe slightly more, just what happened and a general idea of why. (Why is always a complicated thing, I try not to dig too deep into that.)
But I'm still curious as to why this is considered such a big, life-changing event when it was not the first school shooting. Can you help me fill in this part of the puzzle?
While you wait for fresh answers from qualified experts, you may be interested in a related thread.
How long have school shootings been an issue in the US? with a detailed response by /u/veryshanetoday. Please Note: I linked to the section that most applies to your question about Columbine, but their entire thread is filled with great information about the history of school shootings in the US.