The celebration inside the Neuse Christian Academy locker room in Red Springs, N.C., became wild after the high-school team survived a last-second battle against Freedom Christian. That is, until its coach, Ken Adrian, angrily entered the room.

“You guys are all proud and going crazy,’’ he snapped at them. “You guys just beat an eighth grader.”

That eighth grader was Dennis Smith Jr., who had dominated his older opponents. Shooting guard Terry Henderson played alongside Smith at North Carolina State this season and created their share of highlight plays, but the two were opponents in that game five years ago. It remains an indelible memory.

“I’ve been a fan ever since that moment,’’ Henderson recalled. “Checked up on him, saw him grow as a young man. So I’m not surprised now. I saw it coming a long time ago. Throwing the ball off the backboard and dunking it in the eighth grade of a high-school game. He was doing things eighth-graders don’t do.’’

The 6-foot-3, 19-year-old point guard — known by his friends as “Junior” — finished his freshman year at N.C. State and became the NBA draft’s most mysteriously intriguing prospect, as well as getting called its most athletic prospect.

Yet, partly because of N.C. State’s miserable season (4-14 in the ACC), fusses about bad body language, a bad injury (torn ACL before his senior year in high school) and spotty pick-and-roll defense, several mock drafts have him dipping to No. 8, where the Knicks sit.

However, The Post has learned Smith will work out for the Celtics on June 7. Yes, the Celtics, who own the first pick and figure to nab Washington’s Markelle Fultz. Smith already has worked out for Orlando (No. 6), according to sources, and has tentative dates with the Lakers (No. 2) and Knicks.

“He’s going to be one of the great point guards in the league,’’ Henderson said. “Any team that passes him will pay for it later on. If he’s lower than 5, it’s somebody’s loss.’’

N.C. State coach Mark Gottfried, fired along with his staff after a dreadful season that carried large expectations, told The Post recently Smith has “Russell Westbrook athleticism.’’

Ironically, Knicks president Phil Jackson said he is “not interested” in drafting a player who “can jump out of the gym” or “do a triple-double game.’’

Smith is guilty as charged.

Amid the Wolfpack wreckage came two wondrous individual performances by Smith — two triple-double gems, one of which his team lost to Syracuse. It marked the first time an ACC player had posted two triple-doubles against league competition in an entire career. Smith did it in one month of his lone season. And yet neither outing approaches Smith’s signature performance on Tobacco Road.

Henderson’s fondest memory of a lost season was the 20-minute bus ride back from Durham, N.C., sitting next to Smith. The bus glided along Hillsborough Street outside the N.C. State campus, mobbed by fans.

“They were cheering, screaming, had their phones out,’’ Henderson said.

Smith had willed N.C. State to an 84-82 win over Duke, its first at Cameron Indoor Stadium in 22 years. He whipped darts for passes in the final minutes and became unguardable — a 32-point, six-assist magic show on 10-of-18 shooting, 4-of-6 on 3-pointers.

“He was the best player on the floor,’’ Gottfried said.

“He was nasty — evidence he can take over a game,’’ said one NBA scout in attendance.

“It showed what he could do against one of the top teams in the country on the road,’’ Henderson said. “It was his highlight moment.’’

“He did unbelievable individual things,’’ N.C. State assistant coach Orlando Early said, summing up Smith’s season. “There’s a lot of things that go into why a team didn’t have a good year. But the statement he didn’t do things to help the team win? I disagree. He tried to do those things.’’

Duke finished a distant third to N.C. State and North Carolina in the recruitment of Smith, the Fayetteville, N.C., native whose intention always was to play college in-state.

Smith had opportunities to play high school ball at a slew of basketball-factory prep programs around the country, but opted to stay in his hometown of Fayetteville. He moved on from eighth-grade stud at Freedom Christian to Trinity Christian.

“He’s very loyal to family,’’ Trinity coach Heath Vandevender said. “He didn’t want to go away to a prep school because he wanted his family to watch him play.’’

His father, Dennis Smith, was his AAU coach and attended all his practices and games at N.C. State.

“He’s extremely involved, but never got in our way at all,’’ Early said. “He’s not Lonzo Ball’s dad. He’d just watch practice and leave, come to games and leave. It’s not always that way.’’

The female influences on “Junior’’ were his grandmother, Gayle, and Aunt Rhonda. The N.C. State staff never met Smith’s mother, who has dealt with medical issues, according to a source.

Grandma Gayle was a rabid, longtime Wolfpack fan. Her influence on Smith’s pick of N.C. State over North Carolina and Duke can’t be discounted.

“Dennis told me he loved being the underdog,’’ Gottfried said.

For all the college losses, Smith enjoyed only triumphs at Trinity Christian. He owns two state-title rings, though one of the jewels was earned as a de facto assistant coach and early-enrollment college student as he rehabbed his knee.

As a freshman, Smith’s team advanced to the state finals. As a sophomore, Smith made sure the team returned to Fayetteville with the state trophy, sinking two free throws with 1.5 seconds left to win the championship.

Vandevender, though, was more impressed with his response.

“We won it on Saturday,’’ he said. “We’re back to school on Monday and Junior is carrying a basketball inside a trash bag. I asked him what’s he’s doing? He said after school he was going to the gym to work on his ball-handling. He had one day off after winning the states. That’s a different breed.’’

The torn ACL prevented Smith from experiencing a second title on the court. One month before his senior year, in August 2015, Smith tore his ACL in an AAU contest in California.

Well before that, N.C. State’s Early had confirmed Smith’s pedigree at an AAU Christmas tournament in Las Vegas.

The sophomore put up 36 points against a more talented club from Canada that featured Thon Maker, who now plays for the NBA’s Bucks.

“Maker’s team was way better, but you could see it in Dennis’ eyes in warmups,’’ Early said. “His tenacity, toughness, the drive he had to win the game, understanding every shot was going in.’’

N.C. State offered him a scholarship the next day, and Early still refers to it as “The Thon Maker Game.’’

However, 18 months later, Smith’s high-school career ended in an AAU Adidas showcase weeks before going back for his senior year. As many such injuries occur, he took a bad step on a fast break.

“To say we weren’t worried wouldn’t be totally honest,’’ Early said. “But with his unbelievably competitive nature, I knew he’d attack his rehab at a high level.’’

Smith missed his senior year, but not before doing everything possible to sway his way back. Way ahead of schedule on rehab, Smith phoned Early and asked him to research the number of games he would need to be eligible for the famed McDonald’s All-American Classic. All parties convinced Smith, who had become fixated on the game, to let it go. His future was too bright to rush.

Into January, Smith attended almost all of Trinity’s practices, games and team meetings, essentially the club’s point-guard coach.

With the McDonald’s goal torpedoed, Smith came up with another brainstorm while driving in the car with his dad.

“I wish I could say it was my idea, but it was his,’’ Early said.

Having already loaded up with enough high-school credits for graduation, Smith decided to enroll early at N.C. State. He took a full 15-credit course load and attended N.C. State practices, traveled with the team and saw his rehab monitored by the school’s medical staff.

“It was very advantageous,’’ Gottfried said. “He’s always been a student of the game. He wanted to get a head start.’’

Technically, Smith, when he declared for the draft, is a one-and-a-half-and-done.

“I really believe that getting hurt was one of the best things that happened to me before college,” Smith said during his freshman year.

“He’s just a very bright kid,’’ Early said. “I remember him telling me he wanted to finish all his classes so second semester senior year he could chill. [Enrolling early] helped him learn a lot about scouting and watching film at this level.”

The ACC Freshman of the Year has to dispel some myths to NBA executives. Smith’s agitated on-court state wasn’t a good look amid the losing. Early knows it, admitting the visage of a scowling Stephon Marbury is a fair comparison.

“When you’re a point guard, there’s leadership things you need to grow into,’’ Early said. “Dennis doesn’t hide his emotions. When we’re struggling, he can’t hide it. It’s something he has to work on.’’

“I’d rather see a kid mad at losing compared to seeing a kid who goes with the flow and doesn’t care,’’ Henderson said. “It was his competitive nature.’’

The good news is that same eighth-grader throwing balls off the glass in a high-school game hasn’t lost explosiveness since ACL surgery.

“His quickness and athleticism is NBA-ready right now,’’ Early said. “His ball-handling and court vision is NBA-ready. Had we had a good year, he’d be in the conversation for the No. 1 pick.’’