The voices of those first-graders singing in the holiday show — tiny warbles echoing in the huge gym, nearly drowned out by grandpa coughs and chair-leg scrapes — were unforgettable that year.

Usually, I well up when it’s my kid on the stage. But that year, 2012, just a few days after 20 first-graders were gunned down inside Sandy Hook Elementary School in Newtown, Conn., it was impossible not to choke back tears just at the sight of 6-year-olds filing onto the risers.

Counting them, there were 18, 19, oh my God, 20. Exactly.

It was gut-wrenching trying not to envision those 40 small hands going up to shield against a storm of gunfire.

This won’t stand, we thought. It’s the horrific tragedy that will change America.

And it did. But not in a way we hoped.

Six years later, American schools experienced a record-breaking year for carnage, with 94 people shot, 33 of them fatally, according to The Washington Post database on school shootings.

Compared with all the other gun violence, school shootings are rare in number. Our database counted 25 this year.

But what that doesn’t account for is the radiating, collateral damage to an entire generation of children.

There are the witnesses, kids who experience wartime-level carnage in math class. We see what that kind of violence did to generations of soldiers. Imagine an 8-year-old going through shell shock.

These kids are among at least 220,000 students who have been exposed to gun violence at school since the 1999 massacre at Columbine High School, according to The Post’s remarkable series on school shootings. Kids heard screams and the boom of gunfire and experienced the panic of marching with their hands up out of their schools as SWAT teams swarmed the campuses.

Then there are the kids who have been in a school during a lockdown, when a gun was found or a threat was made or a shooter was stopped. That number? More than 4.1 million kids lived through at least one lockdown during the 2017-2018 school year.

One of these was at Silver Spring International Middle School earlier this year, when the school received a report of someone who was in the building with a gun.

Kathleen Isaacson, whose eighth-grader was in one of the classes, said kids were hunkered down, frightened, texting frantic goodbye messages to their parents. They knew what had just happened at Marjory Stoneman Douglas High School in Parkland, Fla.

That night in February, after parents rushed to the school when alerts were sounded and kids had crawled out from under desks, Isaacson’s daughter asked her mom to stay in her room while she fell asleep.

“As a mom, you can’t tell them, ‘Oh, this is never going to happen here,’ ” Isaacson told me. “Because that would be a big, fat lie. It has happened. Kids have seen that these things happen.”

Imagine a nation that used to be determined enough to shield children from violence that we created a ratings system for movies.

Now, we legislatively shrug at the idea that we ask millions of kids to live through a horror movie every day.

The Educator’s School Safety Network estimated that threats or actual violence happen about 10 times a day in American schools. The Post’s research — which included other forms of violence, such as bomb threats — found that about 16 campuses lock down every day, with nine of those ordeals related to gun violence or the threat of it. The Post’s final tally found more than 6,200 lockdowns during the 2017-2018 school year.

But wait, there’s more.

The millions of other children who have managed to avoid living through a lockdown have not escaped emotional trauma thanks to the ubiquity of active-shooter drills. Reading, writing, rifle fire.

Some schools try to calm the kids by telling them to practice hiding from a tiger. Children know the animals are dangerous but also very unlikely to be roaming around a school. Even when it’s just a drill, as kids scrunch up under desks or stay silent and learn to communicate with hand signals, teachers have reported that their students are emotionally traumatized by the acting.

I saw it firsthand. On a weekend after an active-shooter drill at his school, my 11-year-old son ducked for cover in the laundry room, blanching with fear when he heard a loud noise outside. That night, he slept on the floor of our bedroom.

This mauling of a generation has been the nation’s answer to the 2012 tragedy of Sandy Hook. Until now.

Because the calls for change came fast and furious after a gunman with an AR-15 semiautomatic rifle walked into Stoneman Douglas on Valentine’s Day and slaughtered 17 people.

The teenage survivors of that shooting have demanded action — and delivered it. They organized a massive March for Our Lives protest in the spring and challenged gun culture during the midterm elections that ended Republican control of the U.S. House of Representatives.

Legislators in 26 states and the District enacted 67 gun control laws in 2018, according to the Giffords Law Center.

A number of those include red-flag laws, legislation that allows those close to a suicidal gun owner to apply for an emergency judicial order to have the weapons temporarily removed. Remember, school shootings are often suicides that begin with a massacre.

Yes, this has been an especially deadly year in our schools. But if it leads to gun-control laws, 2018 could be forever remembered as the year our nation said “enough.”

And a generation of children won’t have to run from a tiger.

Twitter: @petulad

Read more Petula Dvorak: