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THE LINE OF PEOPLE snakes around the front of Goodwill headquarters in Roxbury. It’s a bitterly cold January evening, but they wait, most holding bags — black trash bags, white plastic bags with smiley faces, paper shopping bags — bulging with clothes. Several tons altogether.

Jared Sullinger, in gray sweat pants, black sneakers, and a black Polo sweat shirt, waves to the crowd and ducks inside, where he will spend a couple of hours handing out autographed pictures, chatting up eager fans, and posing for grip-and-grin photos. On the eve of what will be his final game of the season, he is here to help boost donations, which often ebb in winter. The clothing drive, which Sullinger initiated, is one of the first tests of his celebrity.

Some 600 people come out, including Elizabeth Huff of Londonderry, New Hampshire, and Eric Maxwell of Nashua. “He’s already made such a difference,” Huff, a 25-year-old medical assistant, says of Sullinger’s contribution to the Celtics. Maxwell, a 25-year-old accountant, says he hopes Sullinger develops into Garnett’s successor. Denise Jessamy, a 42-year-old fan from Dorchester, already feels protective. Citing a tussle in a recent game with the Miami Heat, she says, “I was ready to go down to the Garden and fight for him.”

Before greeting fans, Sullinger is led into a conference room to address teenagers in Goodwill’s Youth Initiative. He attempts a joke about girls and makeup, which lands a little flat, and then gets preachy. “Stay in school,” he tells them. “Do your homework.” It sounds like generic public-service-announcement stuff ripped from afternoon TV. But Sullinger has a reason for dispensing it.

He tells them the story: how, as a 15-year-old sophomore at Northland High School in Columbus, he screwed up big. This was 2008. A star on the top-ranked Northland basketball team, he was struggling in the classroom. He was put on academic probation. Unmoved by the warnings, he blew off his work for biology class. His basketball coach had seen enough, and he benched Sullinger for a big playoff game.

That coach was his father, James “Satch” Sullinger, a legend in Ohio high school basketball who held his players to high standards, on and off the court. His philosophy: “You play the game the way you live your life.” Jared tried to negotiate. His father didn’t budge. “I’ll be damned if he was going to turn that program into junk,” Satch Sullinger says now.

It was stunning, the father keeping the son out of such a consequential game. The son, after all, was already one of the top high school players in the country. Jared couldn’t believe it. “My body went into shock,” he told me. “I couldn’t move. I couldn’t speak.” But it got worse. With Jared in street clothes, Northland lost the game, crashing out of the state tournament. “It was a tough night,” says Trey Burke, a close friend and former Northland teammate who’s now a star sophomore guard at the University of Michigan.

The lesson Satch says he wanted to impart was this: “Don’t you ever treat something you love with disrespect again.” It worked. Jared rededicated himself to school. His work ethic improved. He learned what personal responsibility meant. “It’s just like it has cleansed my soul,” he said the next year, when Northland claimed the state title. That stain of failure — of letting his team and his city down, of being the face of disappointment on the front page of the local sports section — is still fresh.

This is why he’s lecturing teenagers at Goodwill on a cold Tuesday evening. “You don’t want that,” he tells them. “That’s the worst feeling ever.”

The story offers a glimpse into how Sullinger was raised, the youngest of three boys in a basketball-obsessed household. His older brothers, Julian and J.J., both played Division 1 college basketball. His mother, Barbara, a math teacher in Columbus, told Cleveland’s Plain Dealer that she had bought her sons blocks, trucks, and other toys when they were young, but “they didn’t want anything to do with anything unless it was round and bounced.”