“Don’t know much about history . . .,” goes the famous song. It’s an apt motto for the Common Core’s elementary school curriculum.

And it’s becoming a serious problem.

A 2014 report by the National Assessment of Educational Progress showed that an abysmal 18 percent of American high school kids were proficient in US history. When colleges such as Stanford decline to require Western Civilization classes or high schools propose changing their curriculum so that history is taught only from 1877 onward (this happened in North Carolina), it’s merely a blip in our news cycle.

A 2012 story in Perspectives on History magazine by University of North Carolina professor Bruce VanSledright found that 88 percent of elementary school teachers considered teaching history a low priority.

The reasons are varied. VanSledright found that teachers didn’t focus on history because students aren’t tested on it at the state level. Why teach something you can’t test?

A teacher I spoke with in Brooklyn confirmed this. She said, “All the pressure in lower grades is in math and English Language Arts because of the state tests and the weight that they carry.”

She teaches fourth grade and says that age is the first time students are taught about explorers, American settlers, the American Revolution and so on. But why so late?

VanSledright also found that teachers just didn’t know enough history to teach it. He wrote there was some “holiday curriculum as history instruction,” but that was it.

Arthur, a father in Brooklyn whose kids are in first and second grade at what’s considered an excellent public school, says that’s the only kind of history lesson he’s seen. And even that’s been thin. His second-grade daughter knows George Washington was the first president but not why Abraham Lincoln is famous.

As the parent of a first-grader, I’ve also seen even the “holiday curriculum” in short supply. First grade might seem young, but it’s my daughter’s third year in the New York City public school system after pre-K and kindergarten. She goes to one of the finest public schools in the city, yet knows about George Washington exclusively from the soundtrack of the Broadway show “Hamilton.” She wouldn’t be able to tell you who discovered America.

So far, she has encountered no mention of any historical figure except for Martin Luther King Jr. This isn’t a knock on King, obviously. He’s a hero in our house. But he can’t be the sum total of historical figures our kids learn about in even early elementary school.

For one thing, how do we tell King’s story without telling the story of the Founding Fathers, the Constitution or of Abraham Lincoln? King’s protests were effective because they were grounded in the idea that America was supposed to be something specific, that the Constitution said so — and that we weren’t living up to those ideals.

The Brooklyn teacher I spoke with says instructors balk when it comes to history: They don’t want to offend anyone. “The more vocal and involved the parents are, the more likely the teacher will feel uncomfortable to teach certain things or say something that might create a problem.” Which leaves . . . Martin Luther King.

She cited issues around Thanksgiving, like teaching the story of pilgrims and the Native Americans breaking bread together as one that teachers might sideline for fear of parents complaining. Instead of addressing sticky subjects, we skip them altogether.

As colleges around the country see protests to remove Thomas Jefferson’s statues from their campuses, it’s becoming the norm to erase the parts of history that we find uncomfortable. It’s not difficult to teach children that the pilgrims or Thomas Jefferson were imperfect yet still responsible for so much that is good in America.

Jay Leno used to do a segment on his show called “JayWalking,” where he’d come up to people on the street and ask them what should’ve been easy historical questions. That their responses were funny and cringeworthy enough to get them on the show tells you how well it went.

Leno never asked the year the Magna Carta was published or when North Dakota became a state. He would ask what country we fought in the Revolutionary War, to name the current vice president or how many stars are on the American flag. And yet adults had no idea.

We talk often about how fractured our country has become. That our division increases while school kids are taught less and less about our shared history should come as no surprise.