I don't know when you were a kid, but there have been a metric ass-load of school shootings in the U.S. over the years:
http://en.wikipedia.org/wiki/List_of..._United_States
Columbine may have been the one with the most fatalities at a high school up to that point, but that's just a matter of scale - it wasn't fundamentally "new" or "unique".
Columbine probably sticks in our mind more for the news coverage than any other reason, since it happened in the era of the 24-hour news cycle AND the World Wide Web.