Scissors. Calculators. Books. Terrified students at a Michigan high school grabbed anything they thought they could use to defend themselves as they hid from a gunman who had just opened fire and now stalked them. “You never think it’s going to happen to you. And then it does,” said one student. “It’s just insane.”

The horror — the insanity — that unfolded at Oxford High School on Tuesday is sadly familiar to Americans, who have become too accustomed to such bloodshed. “A routine American tragedy” is how The Post’s reporters characterized the agonizing handful of minutes in which shots rang out at the suburban Detroit high school.

Four students — Hana St. Juliana, 14; Madisyn Baldwin, 17; Tate Myre, 16; Justin Shilling, 17 — were killed. Seven people — six students ranging in age from 14 to 17 and a 47-year-old teacher — were injured, some critically. Michigan authorities identified the alleged gunman as Ethan Crumbley, a 15-year-old sophomore at the school, whom police took into custody within about five minutes of the first 911 call. He was charged Wednesday as an adult on multiple counts, including terrorism and first-degree murder. Prosecutors are also considering charges against the suspect’s parents.

“I’ve seen some of the actual video of the shooting itself, and it’s clear he came out with the intent to kill people,” Oakland County Sheriff Michael Bouchard said on CNN. “He was shooting people at close range, oftentimes towards the head or chest. It’s just absolutely cold-hearted, murderous.” He said the suspect fired through barricaded classroom doors and tried to breach them. The teachers who bravely and calmly acted to protect their students deserve thanks, as do the police and other first responders who worked to save lives.

But it is disgraceful that Americans have accepted massacres as inevitable — Tuesday’s school shooting was the 28th this year and the deadliest — to the point that students spend so much of their time in active shooter drills rather than enjoying what school should be about — learning, socializing and growing.

Why this young man, as has been alleged, picked up a gun and went on a rampage against his schoolmates is a question that authorities are seeking to answer. That he appears to have used a handgun his father purchased on Black Friday — a 9mm Sig Sauer SP2022 semiautomatic — underscores once again the danger of unsecured firearms and the need for common-sense gun-safety laws. It’s time — long past the time — not just to prepare students for mass shootings but to prevent them.