When the Signature School of Evansville, Ind., became a public charter campus, the district it had been part of dispatched technicians to cut phone and Internet lines and remove computers and a copy machine.

When that didn’t stop the breakaway effort, the superintendent told Signature teachers they had to return to their former schools. “Everyone was quiet because we were in tears,” said math teacher Shannon Hughes. “We felt we were going to lose our jobs. Our health insurance was in doubt. What would happen to us?”

American charter schools often have rough beginnings. School districts usually resist what they see as damaging competition for students and resources. But Signature’s recovery from troublesome days in 2002 and 2003 has been exceptional. It has become one of the nation’s most successful high schools, despite being far from the big metropolitan areas that produce the most celebrated campuses.

Signature has been highly ranked on my annual Challenge Index list of high schools for many years. On the 2019 list, it is for the first time No. 1.

The list measures participation in college-level Advanced Placement, International Baccalaureate and Cambridge exams. Signature has done well by replacing the typical high school curriculum with AP and IB courses from ninth grade on, while remaining open to all students, with a random lottery whenever it is oversubscribed.

According to a 2013 study by Stanford University’s CREDO research organization, about three-quarters of charters do only as well or less well than regular schools. But the quarter of charters like Signature that do significantly better — as well as many other schools that aren’t charters but harbor similar ambitions — show how to unleash the academic potential of U.S. teenagers.

Robert L. Koch (pronounced cook), head of a family metalwork company in Evansville, helped start Signature in 1992 as a half-day enrichment program for 11th- and 12th-graders. He had been dissatisfied with the way his children and employees were being educated. He could not get the superintendent to show him the average SAT scores. When a district staffer finally slipped him the numbers, they were not good.

A new superintendent agreed to help him do something. They joined with other local leaders to support standards above what was called the “general lane.”

“We did a video showing that if you are in the general lane,” Koch said, “you will probably be in the unemployment lane.”

The superintendent persuaded the school board to authorize a special campus called Signature for enhanced and advanced high school courses, open to public and private school students. Space was found in an old downtown retail store. It was popular with families and teachers, but the local newspaper called it elitist. After 10 years, principals at traditional schools said they needed the money being spent on Signature.

The superintendent who had helped start it moved to another job. The educators, businesses and parents who wanted to keep it decided to make it a full-time high school under Indiana’s new charter law.

Hughes, one of its first teachers, said she found it invigorating. The AP courses demanded much more than Evansville schools had before. As the AP calculus exam — written and graded by outside experts — approached, she held individual preparation sessions for her students on weekends. Over the years, the school switched from regular courses to AP versions of history, English and science. It added IB courses in 2006.

“The teachers and I wanted to create an environment where academics came first,” said Vicki Snyder, Signature’s first principal. Signature has sports (track, golf and cross-country) and some music, speech and service activities, but nothing close to what regular public high schools have.

Only 14 percent of Signature’s 366 students come from low-income families. The school is trying to recruit students from poorer parts of town. Other local schools appear to be inspired by its example. Evansville Central High, where 53 percent of students are from low-income families, is now in the top 10 percent of U.S. schools, according to the Challenge Index list.

The rise in standards “is going to pay huge dividends in the long term in the economy and future of southwest Indiana,” Koch said. Hughes said the students absorb “a culture of high expectations. They help each other.” Signature students take at least 12 AP or IB courses and exams before graduation.

If educators in an otherwise average city like Evansville can make such big improvements, why not everyone else?