Andrew Pollack calls the Parkland, Fla., school shooting “the most avoidable mass murder in American history.” Avoidable because of how the system refused to react rationally to years of warning signs about Nikolas Cruz.

And the book Pollack, whose teen daughter was one of Cruz’s victims, has written with Max Eden shows that the same failure could occur almost anywhere in America.

Excerpted in Monday’s Post, “Why Meadow Died” reveals educational records, like teacher daily diaries, tracking Cruz’s flagrantly psychotic behavior from an early age.

A private pre-K program expelled him at age 2 for biting other tots. A kindergarten teacher reported that he “seemed to identify as an animal,” citing his crawling on the ground, pouncing on other students, growling and, again, biting.

But early on, he got diagnosed as suffering “developmental delay,” “language processing deficiency” and “attention deficit disorder.” And such “emotional and behavioral disability” diagnoses, the authors explain, guarantee that their subject becomes a kind of untouchable in modern US schools.

Under the Federal Individuals with Disabilities Education Act, “children with disabilities must be educated with children without disabilities to the maximum extent possible in the least restrictive environment.”

“Full inclusion” in ordinary classes becomes imperative. And, noted one teacher, “No one ever challenges an [emotionally disabled] parent or kid because it’s not worth the risk of litigation.”

No matter that he got worse. One middle school teacher recalls his “screaming in the hall.” “If something frustrated Cruz, he would curse and threaten anyone nearby,” the authors write. And: “For no apparent reason he would burst into maniacal laughter.”

He told an eighth-grade teacher, “I’m a bad kid. I want to kill.” Cops were called to the Cruz home 39 times over six years. Yet no one ever invoked the “danger to himself or others” standard to try for an involuntary mental commitment.

As Eden puts it, “All the things that were going wrong in American public education joined forces to let him slip through every crack.” And so, his record unblemished by criminal or mental health red flags, Cruz legally bought his murder weapon soon after turning 18.