On the second seismic night almost six years ago now, when President Obama was reelected, a number of digital outlets noted an eruption of tweets maligning the president, the son of a black Kenyan man, for his race. They counted that most-vile of racial epithets and the most-nauseating of racist imagery.

More shockingly, many originated from the Twitter accounts of high school students, who Jezebel.com’s Tracie Egan Morrissey observed “. . . feature their real names and advertise their participation in the sports programs at their respective high schools.”

It all raised, among myriad questions, the role of schools, and even their athletic teams, in teaching tolerance in the present with the hope of extinguishing hate in the future.

That’s precisely why we should be concerned by, and not dismissive of, Tuesday night’s revelation that Josh Hader, one of the pitchers showcased in Major League Baseball’s 89th All-Star Game, was a serial hate tweeter as a star athlete at Old Mill High School in suburban Baltimore’s Anne Arundel County.

Hader was one of those teenagers back then. He was 17 on the eve of President Obama’s reelection. And it was as a 17-year-old when Hader started using Twitter to send shout-outs for white power and the KKK from his hometown of Millersville, Md., a little south of Baltimore-Washington International Marshall Airport. At that time, he expressed abhorrence for those who are gay, regurgitated that well-worn slur for black folk and spat misogyny.

Hader is 24 now. After the All-Star Game, he quickly shuttered his Twitter account when the disgusting tweets, next to an avatar of him in his Milwaukee Brewers jersey, were reposted. He gave a mea culpa. The Brewers added an apology. Baseball announced it was sending Hader to sensitivity training.

And to be sure Hader was summarily absolved, some of us in the media found teammates of color to stand up for Hader.

It was little more than the pro forma theater that we’ve come to expect these days after an athlete or other celebrity, particularly those who are white like Hader, is found to have shown a side of themselves that we are told to believe is fake at best or misguided by youth at worst.

Just this year, on the eve of the NFL draft in April, Wyoming quarterback Josh Allen had to explain racist tweets he typed as a high school star. He and everyone around him apologized and said his past wasn’t who he became. Shortly before, Donte DiVincenzo, Villanova’s star guard in the Final Four, had to delete old tweets from his teenage days because they were laced with racial and homophobic slurs. They reflected black music — rap — he listened to, not him, it was explained.

It also can’t be ignored that this country has a president who has turned his Twitter account into a truncheon to attack people of color, immigrants and women with unchecked impunity that has emboldened others, including youth, to do the same.

I don’t know Hader or Allen or DiVincenzo. But I do know that if we’re going to be a better society going forward, we need to be better teachers of right and wrong from the start. That is why an apology or assigning blame to youthful ignorance is neither enough nor right-headed. Dismissing such behavior now, and for good, is tilling the soil for racist, homophobic and misogynistic behavior to propagate.

Hader doesn’t need to sit before a computer screen and take a multiple-choice test on respectful behavior toward black people, LGBT people and women in the workplace. Instead, he should be dispatched back to his hometown of Millersville, which is 71 percent white, and to the Anne Arundel County schools he attended, to tell those teenagers how what he did just a few years ago when he sat at those same desks was a misstep. How wrong he was. How it turned what was supposed to be a celebratory evening for him on a national stage into a nightmare of an embarrassment. How he had to explain to his employers, his teammates — several of whom are of color — and his fans what he had done.

Hader’s behavior isn’t isolated to him. It has repercussions. It festers. It metastasizes. To be sure, it still exists in the school system that produced him. Just a few months before the last academic calendar year recessed for the summer, the Anne Arundel NAACP complained that black students in schools there were targets of daily racial abuse. Anne Arundel also produced former major league catcher Steve Clevenger, who was suspended in 2016 while with Seattle for racially insensitive tweets about a police shooting victim who was black and the Black Lives Matter movement that protested such tragedies.

“Since last October, we’ve been tracking incidents of hate in schools,” Maureen Costello, the director of Teaching Tolerance with the Southern Poverty Law Center in Montgomery, Ala., told me by phone Friday. “Every month, there is exactly this sort of thing, tweeting out . . . a lot of racist stuff, clueless white kids thinking it’s a big joke.”

This is why the Southern Poverty Law Center in the immediate wake of President Trump’s racially charged election responded to teachers’ suddenly escalated concerns about classroom atmospheres reflecting the election with recommendations for instruction going forward. One suggestion was to “encourage everyone in the school community to be aware of bullying, harassment and bias in all their forms . . . to let staff and students know that you expect them to speak up when they see or hear something that denigrates any member of the school community. When students interrupt biased language, calmly ask questions, correct misinformation and echo others who do the same, they send their peers a clear message: This kind of language doesn’t fly here.”

“It’s the impact,” Costello emphasized, “not the intent that matters. It’s kind of a public health issue. It’s damaging to a community. Our philosophy is not about punishing, but reconciling with the people [offenders] hurt. We’re never going to fix this unless people realize it hurts other people.”

Which is why the Josh Hader incident is exactly what we call a teachable moment.

Kevin B. Blackistone, ESPN panelist and visiting professor at the Philip Merrill College of Journalism at the University of Maryland, writes sports commentary for The Post.