Another book ID one.

[Adreno]

It’s a book about a school massacre, set in America.

It’s told from the mother’s perspective. She’s very wealthy, never really wanted children, and she’s left to pick up the pieces now that her son is in jail.

I can’t remember all the details but the son locks people up in the school gym, chains the doors closed.

It’s not We Need To Talk About Kevin, but quite similar.

Is it the Jodi Picoult one? Nineteen Minutes hers is called I think.

Is it Every Last One by Anna Quindlen?

Actually scrap that I think I'm getting confused with another book I've read, which I think could be the one you're looking for. Now I just have to try and remember what it was called....

Three Hours by Rosamund Lupton is what I was thinking of