That was in 1998.
Today at 28, Hart lives in Tokyo and makes his living singing soulfully in Japanese across Japan. Winning a prime-time television music competition last year allowed him to quit his day job as a vending-machine serviceman.
Last week Hart returned, with an entourage, to the classroom of his Gunn High School Japanese teacher, Yukie Hikida, to share his unlikely story.
He now banters easily in Japanese with Hikida, but the teacher remembers him as a student who was "so quiet and polite, but not studious."
Hart is blunter, reminding Hikida that he was a "terrible" student who managed to graduate only by taking a high-school equivalency exam.
He entered Foothill College at 16, where he continued studying Japanese and began a series of jobs bagging at Safeway, refueling private jets at SFO, working customer service for a Japanese skin-care brand with a notion of somehow getting back to Japan.
A break came in 2009 when San Francisco-based automated retailer Zoom Systems, where his mother and sister worked, urgently needed a vending-machine technician and sent him over.
Hart began vocal training for the first time only after arriving in Japan, though he'd studied flute as a student at Fairmeadow Elementary School and later picked up clarinet, oboe and saxophone.
Posting his videos on YouTube led to one thing and then another including meeting the woman he married last year, singer Hitomi Fukunaga, a Japanese national who performs in English. Eventually the YouTube videos and television shows led him to the national contest that launched his full-time singing career.
Today Hart finds himself performing in front of weeping fans, and on the receiving end of gifts that include flowers, CDs, food, jewelry and remarkably personal letters.
"I'm singing a song and the woman in front of me is just bawling, and I'm trying to understand what the audience is relating to in my songs and how can I do that better," he told the Gunn students. "I'm going to try to make them laugh, too.
"Performing is a human-to-human interaction and nothing matters more than your ability to connect with your audience. There are a lot of performers with good musical skills that don't go anywhere."
With a manager and a record label, Hart now spends much of his time on interviews with Japanese media and promotional tours.
"It's a little unusual because I'm a foreign artist and that's kind of a bit of an extra appeal. They want to know how I got to Japan, what were the steps.
"Honestly, I spend more time talking about singing than actually singing."
But upon request, Hart took a moment to sing a few solo lines in Japanese for the Gunn classroom.
Hart said his parents, both musicians among other things, "were always supportive, but always worried" about his future.
"Having a mom who went to Stanford and then not going to Stanford that's not fun," he said. "They always supported my music and always supported my Japanese study, but it was stressful."
He advised the Gunn students to take risks and seek out challenges in their lives.
His own newest challenge is the goal of making music sustain him for the long term, he said.
"Instant celebrity isn't fulfilling it doesn't help you in the long term," he said.
"I want to make sure the connection with the audience stays strong, to keep making songs that are powerful and meaningful for them.
"I want to make sure that in the long term I'm doing something substantial."
Although he was a poor student, Hart said JLS and Gunn set him on a good path.
"Despite my lack of enthusiasm, the teachers were great and instilled in me a lot of great skills that allowed me to cope in the real world," he told the students.
"It's cheesy when I say it but, honestly, looking back, this is a great place to start from.
"You're in a great position, at a great school."