St. Thomas More, an all-boys boarding school with an enrollment of 125, certainly doesn’t look as if it would have any real importance to basketball’s elite.

Nestled on 110 acres overlooking a tree-lined lake in southeastern Connecticut, it has three dormitories, three classroom buildings, a library, a chapel, a modest gymnasium and little nightlife. The nearest town is a 12-minute drive. At 10:30 each night, the WiFi is shut off and students must head to bed.

“Not going to lie, it was pretty bad in terms of your social life,” said Warriors forward Eric Paschall, who spent his senior year at St. Thomas More a half-decade ago. “But in terms of what you needed with basketball and academics, it was perfect.”

As Golden State tries to maximize a transitional season, it is relying on three St. Thomas More alums — Paschall, guard Damion Lee and forward Omari Spellman — for major minutes. None of their stays at the tiny prep school on Gardner Lake overlapped, but Lee (2010-11 school year), Paschall (2013-14) and Spellman (2015-16) each considers his time there critical to his NBA journey.

St. Thomas More, which boasts an average class size of eight, offered Paschall, Spellman and Lee a distraction-free environment to focus on their grades. They read 30 books in a year for a world literature class, attended supervised evening study halls every Sunday through Thursday, and heard a knock at their door whenever a teacher wanted a late assignment.

On the court, Lee, Paschall and Spellman were asked to test their limits. They awoke at 5:30 a.m. for 6 o’clock practices, took buses to prep schools throughout New England, and competed against teams loaded with Division I recruits.

“One thing that Damion, Eric and Omari all had in common was that they lived in the gym,” said St. Thomas More head coach Jere Quinn, who relies almost entirely on word of mouth for recruiting. “Of course, if you don’t live in the gym, why come here? There isn’t exactly much else to do.”

Quinn, who has won more than 1,000 games and mentored 10 NBA players in his 42 years with the Chancellors, let Lee, Paschall and Spellman know early in their stays that he would hold them to a higher standard than their less-talented peers. When Lee, Paschall or Spellman jogged during wind sprints or failed to box out their men, Quinn reminded them that they could make a lot of money playing professionally if they stayed diligent.

This was news to Lee and Paschall, who had both arrived at St. Thomas More as relatively unheralded prospects. Having Quinn, a former Basketball Hall of Fame nominee, tell them they had a chance to reach the NBA made them only more willing to stay later after practices to get up extra shots and review the upcoming game plan.

“He was one of the first coaches who really believed in my potential,” said Lee, who still remembers being a sophomore on junior varsity at Mount Saint Joseph High School in Baltimore when a coach told him he’d be lucky to land a Division II scholarship. “For a young kid, I can’t tell you how much that meant to me.”

Lee, who had enrolled at St. Thomas More for a post-grad year after backing out of his commitment to Towson, was the top running mate for Andre Drummond — now a two-time NBA All-Star with the Pistons — on a Chancellors team that won the school’s only prep-school national championship, in 2011. Three years later, Paschall earned conference-player-of-the-year honors before St. Thomas More fell to current Utah guard Donovan Mitchell and Brewster Academy in the prep-school national title game.

Spellman also received conference-player-of-the-year honors for a team that lost in the national championship game. But unlike Lee and Mitchell, he had come to the scenic campus in Oakdale, Conn., in 2015 as a five-star recruit with a reputation for bad eating habits and uneven grades.

During one of his first meetings with Spellman, Quinn told him that he had as much upside as anyone Quinn had ever coached — that is, as long as Spellman learned how to be professional.

In his nine months at St. Thomas More, Spellman started earning grades that matched his IQ (he scored a 1,250 on the SAT), discovered a passion for poetry and made strong friendships. To this day, he still has a group text with his former St. Thomas More teammates and goes on summer vacations with them.

“It’s basically just a bunch of dudes by a lake with buildings, but it was exactly what I needed at that time in my life,” Spellman said of St. Thomas More. “I’m so grateful for my time there.”

Quinn checks the Warriors’ box scores online the morning after each game. Every Thanksgiving and Christmas, he texts Lee, Paschall and Spellman that he’s thinking of them.

In three weeks, while in San Francisco for the Pistons-Warriors game at Chase Center, Quinn will reminiscence over dinner with his former players about the baby-faced teens who used to trade college gear with the hall supervisor for the WiFi code and sneak out of their dorms to pull pranks on teammates. This will be a special reunion for Lee, Paschall and Spellman, who had worried just a few months ago about their old coach’s health.

In the spring, Quinn was hospitalized with Guillain-Barre syndrome, which causes the body’s immune system to attack the nerves and left him bed-bound for weeks. Now back on the sideline, he has guided St. Thomas More to a 7-2 record.

“It’s amazing to see what he’s truly in it for,” Lee said of Quinn, who spurned college and NBA offers earlier in his career to stay at St. Thomas More. “It’s just about helping guide boys to men in Oakdale, Conn. That takes a very special person.”

Connor Letourneau is a San Francisco Chronicle staff writer. Email: cletourneau@sfchronicle.com Twitter: @Con_Chron