Classmates often know when a student is ready to strike back. Parents hear rumblings and have accurate gut sensations.
I read a few articles about the Ohio shooting and all of the students, some of them friends of T.J, said they did not see this coming. He was described as quiet and private, but not angry. Besides the poem he wrote on his facebook page back in December, there does not seem to be any indication of "striking back".
And this is true for all the school shootings: it was always a shock. Parents, families, teachers, classmates, and community members are always left reeling in the aftermath. Even with warning signs, they aren't always... almost never, really... noticeable. It isn't until people look back they start to notice things.
So anyway, no... I don't agree with the author on that part. Other than that, it's a fairly well written article.