It's not like school shootings were unheard of before Columbine -- there had been two the year before. But the nice thing about modern civilization is that we don't and shouldn't have to assume that every weird noise is made by a fellow human trying to kill us.
"A moment later, [a teacher] ran in. She was completely frantic. Everyone just kind of looked at her like she was part of some elaborate prank. But she kept insisting, 'Get under the tables, get under the tables,' and all of a sudden it clicked and everyone scrambled."
"Get out of the library" might have been better advice, but there was no way of knowing at the time.
The fact that Amanda was still alive to talk to us mostly came down to luck and a good choice of table.
"I got under my table, and part of me was still thinking it was some kind of joke, because I couldn't believe it. Then I'm thinking, 'OK, this table is visible from two sides, so I better move.' There was this long table directly across from me that had cubbies underneath, so you could only see them from one side. So I crawled in next to this other girl, pulled a chair in front of me, huddled under there. And then I heard more popping noises, but coming from inside the school. [Then] I heard two very angry voices, they came straight to the back of the library, about 10 feet from me. One yelled 'Everybody get up right now or we're going to blow your [fucking] heads off.' No one got up, so they started shooting."
It was pretty clear that standing up wasn't going to get you sent on your way with a friendly wave.