To date, Teacher Synergy, the company behind the site, has paid about $175 million to its teacher-authors, says Adam Freed, the company’s chief executive. The site takes a 15 percent commission on most sales.

A former chief operating officer of Etsy and former director of international product management at Google, Mr. Freed is a veteran of data-driven growth companies. By selling tens of thousands of items, he says, 12 teachers on the site have become millionaires and nearly 300 teachers have earned more than $100,000. On any given day, the site has about 1.7 million lesson plans, quizzes, work sheets, classroom activities and other items available, typically for less than $5. Last month alone, Mr. Freed added, more than one million teachers in the United States downloaded material, including free and fee-based products, from the site.

“If you have a kid in school in America, they are interacting somewhere with TeachersPayTeachers’ content,” Mr. Freed said in an interview last week at the company’s headquarters in Manhattan.

Mr. Freed took the helm of Teacher Synergy in 2014. One of his first tasks was to bring the technology behind the homespun company up to date without introducing radical changes that might upset its following. That goal has become more urgent now that TES Global, a British company with its own teacher-to-teacher marketplace, has entered the American market.

Last week, for instance, TeachersPayTeachers introduced an iPhone app from which educators can buy materials. The app replaced an older version that allowed users to look up products but, oddly enough, not to purchase them.

“We were not a technology company until very recently. We were a teaching marketplace with a technology underlay,” Mr. Freed said. “Now we are trying to be both.”

The site’s popularity with teachers reflects the convergence of a number of trends in education and technology.