Stephen King book?

Yeah, this book was written at the beginning of his career as Richard Bachman, and the books that he had written at the time under that pseudonym were highly caustic and loathsome. This book predated the Columbine massacre by more than two decades, and this was the one of the four that wasn't individually rereleased around the turn of the century, out of respect for the victims of the tragedy. If this had been after September 11th, The Running Man wouldn't have been rereleased, either.
These are also considered some of his best paced works. Regardless, out of the four books, three end with the protagonist dying violently, while one merely ends with the main character being institutionalized. The Running Man and The Long Walk have protagonists that suffer unspeakably throughout the course of the story (both set in the future and leaning more toward science fiction), while Rage and Roadwork are set more at present times and have characters that merely suffer extreme mental anguish and respond with murder, sabotage, taking hostages, etc. These books rail against the system in a way that is not typical of King's work, and all four would make great movies, but don't hold your breath.

Rage, by Stephen King, had very little to do with school shootings.
Yes, there was a shooting in it, and quite a pointless one, when you
come right down to it, but it was hardly the focus of the story.

To say that 'Rage' gives school shooters a bad name is kind of silly.
I'd say they give themselves a bad name, and their trying to say that
King's book inspired them is pretty weak.

Rage has violence, yes. One extremely pointless act of violence, and
a number of incidents, certainly. The main character is, after all,
in his own words, losing his mind.

The class full of hostages could be said to be suffering from Stockholm
Syndrome (or whatever it's called), but ultimately, the characters in
the story, with two exceptions, learn things about themselves, for better
or for worse, that are the stimulii for some life-affecting decisions.

Personally, I would have loved to see some kind of 'outside' sequel, to
follow the book, rather than just the clipped and edited letters hinting
at the things the class got out of that day, and presumably, the rest of
their lives.

The Bachman books were different from the King books, and Rage was, and is
my favourite of those I've read.

I think it would not translate well to film, though, because such character
driven movies rarely do well. Particularly when editing always seems to
remove the many sub-texts and there's just no way to see the thought process
of the protagonists in film.