Sal Khan, whose online Khan Academy serves up video tutorials to more than 6 million students worldwide each month, wants to reassure teachers that the free educational service isn’t out to take their jobs—nor is it a statement about a teacher’s ability to deliver a lesson effectively.

On the contrary, Khan said, teachers who are using the service with their students feel more empowered than ever.

“It liberates the classroom,” he told attendees of the National School Boards Association’s 72nd annual conference in Boston, “and teachers’ creativity comes out.”

Warm and engaging, with a self-deprecating sense of humor, Khan discussed his nonprofit venture during an April 22 keynote speech to a few thousand school board members, who interrupted him with frequent rounds of applause.

Khan Academy now contains more than 3,100 free video tutorials, mostly on math and science—but the site has begun expanding its scope to other subjects, too...