A running joke emerged between the boys and girls in the 2008-09 second-grade class at the prestigious Trevor Day School in uptown Manhattan.

It was directed at one group of boys, a friend-group bonded by their Yankees fandom.

As such, they wore a Yankees jersey or shirt to school nearly every day.

It led to girls in the grade, as kids that age do, joking that the boys were in a “How long can you go without washing your Yankees shirt?” competition.

For one member of the group, however, the obsession was prescient — Anthony Volpe, the Yankees’ burgeoning rookie shortstop.

“I don’t know if they had a million Yankees shirts, or if they just wore a different one every day or the same one every day,” Gabrielle Smith, who was in both the same pre-K and second-grade class as Volpe at Trevor, told The Post. “But definitely what I remember most is that he wore a Yankees shirt literally every day.”

Fourteen years later, Volpe is again wearing a Yankees shirt or jersey nearly every day, although this time around they’re assuredly getting laundered daily.

After a slow start to his rookie season, the 22-year-old Volpe enjoyed a strong finish to the first half of the year heading into the All-Star break.

Formerly the team’s top-ranked prospect, per MLB Pipeline, Volpe posted a .300/.364/.538 slash line with a .901 OPS and four home runs across his last 24 games before the break.

It raised his season totals to .216/.287/.395 with a .682 OPS and 13 homers. He also has 16 stolen bases.

Volpe was born in Manhattan and spent his formative years in the city before he and his family moved to Watchung, N.J., when he was in fourth grade.

Though he did not graduate from Trevor, the pre-Kindergarten-through-12 private school is where Volpe began his education and development.

Or more specifically, developed into a kid who had one goal in mind.

“It’s special,” Volpe told The Post. “When I was that old, you got a lot of projects or homework or whatever — what do you want to be when you grow up? I guarantee all the stuff I said was to be a Yankee.

“It was a dream, I just loved to play baseball. Being that young, I feel like if you like to play baseball, naturally, you want to grow up to be a baseball player. I didn’t know how anything worked, from Point A to Point B. I didn’t know that there was even a draft, I really didn’t know.

“Being that young, that’s where everyone kind of found their love and passion for the game.”

In the classroom, Smith distinctly remembers their second-grade class — with Mr. Chisty and Ms. Stephen — recorded a music video for a class project.

It was to Bob Sinclar’s 2006 hit “World, Hold On.”

Of course, an energetic Volpe found his way to the front of the video.

Then there was gym class or Physical Education.

Again, an energetic Volpe was the center of attention.

“You know how they joke about those kids who treat elementary school P.E. like it’s the Olympics,” Smith said. “He was definitely one of those kids.”

Beyond his energy, though, it’s Volpe’s character and demeanor that most highlight.

Just like his Yankees attire and youthful energy, it’s stayed consistent.

“What’s so exciting about it is, anytime you mention his name, people smile and they only have good things to say,” Dan Feigin — who serves as Trevor’s Head of School, taught in the school when Volpe was there, and is an alum — told The Post. Feigin’s twin brother, Peter, is the president of the Milwaukee Bucks. “And most of them knew him as a young kid. Knew him as a really happy, friendly, kind, young person.

“We took pride in the kid he was. And now, we love taking pride in what he’s become.”

Volpe believes he’s still the same kid he was at Trevor.

“I feel like nothing’s really changed since then,” Volpe said. “I just love playing baseball. Whatever happened, happened from there. It never felt like anything more than that, and it still doesn’t. I think that’s what’s pretty cool.

“Those are obviously really early years of learning. I remember they made it a lot of fun, we all looked forward to going to school every day.”

When the final bell rang every day and Volpe’s school day ended, he often bolted for baseball fields in the neighborhood, whether it was for Little League or pickup.

Before speaking with The Post following a win this month over the Cubs, Volpe coincidentally was reminiscing about some of those fields with Isiah Kiner-Falefa, whose locker is next to his.

Kiner-Falefa is Volpe’s father Michael’s favorite player, an interesting dynamic given Volpe won the starting shortstop job over Kiner-Falefa during spring training.

Every day, as Volpe was also explaining to Kiner-Falefa, he is provided a reminder of his earliest roots.

“We were kind of just talking about it, even on my way to the field every day, on the West Side Highway I played at all those fields growing up,” Volpe said. “It’s pretty special, and a pretty cool experience. I feel like not that many people can say that on their way to their home ballpark, that they drive by the fields that they played on when they were little.”