I have seen a fair amount of people saying they found this unrealistic in this day and age. I don't. I went to school in the age of Columbine. It occurred just prior to my starting middle school, and heralded in the age of the fake bomb threat. It happened so frequently that complacency just became the standard; after five or six times in a semester people just stop believing it's real until an actual shooting happens again. I distinctly remember a time when my middle school principal got taken to task after he only evacuated the building for five minutes after a threat. Unfortunately, it would be hard for a small town to really feel that something was genuinely wrong, and the way it's presented in this book the administration couldn't do a darn thing anyway.
That being said, it's harder to suspend belief about the fact the everyone in that auditorium has a phone on hand. One or two people saying it, oh hahaha not funny. But you'd be getting a hundred plus reports for this. Even in a small town, law enforcement would be getting there faster; certainly faster than some track stars running down the road to get them.
Nonetheless, I think this narrative felt genuine enough. I found the tweeting interstitial pieces somewhat distracting and hard to track, but I do hate twitter so very much. The individual narratives I actually found to be an interesting perspective. I don't think they lacked development; instead, if you really put yourself in the first person emotionally they said a lot about how that person was experiencing that situation. Nonetheless, the overall story lacked a certain amount of development that perhaps some more backstory could have contributed to.