GAINESVILLE, Fla.—It was a sticky Thursday afternoon in the middle of summer break when dozens of teenagers walked through the doors of their high school. One of the world’s most dominant teams was about to start math practice.

There was probability in one classroom and pre-algebra next door, code-breaking down the hall and number theory around the corner. And there were few adults to be found anywhere. The students would spend the rest of the day teaching each other.

I had also come here to learn from them. I wanted to understand how this otherwise average public high school in Florida had managed to win 13 of the last 14 national math championships.

The Buchholz High School math team is a dynasty built by one teacher with a strategy for identifying talent, maximizing potential and optimizing the American system of education.

Will Frazer popped out of his flaming red Corvette as his students were trickling into the classrooms. A bond trader on Wall Street in the 1980s, Mr. Frazer retired young and moved to Florida, where he became a scratch golfer and lived the dream for a decade. Then he got bored.

He took a job at Buchholz coaching golf, switched to teaching math, quickly formed a math team, applied the lessons of his experience in finance and turned a bunch of teenage quants into a fearsome winning machine.

“The difference between what I do now and what I did on Wall Street is that I used to get paid money,” said Mr. Frazer, 63. “Now we get trophies.”

The extraordinary thing about the Buchholz math team is how ordinary Buchholz is. It’s ranked 66th among schools in Florida and outside the top 1,000 across the country, according to U.S. News & World Report. But at the annual championships of Mu Alpha Theta, the national math honor society, the Buchholz kids have trouble counting the shiny objects they lug home.