ORANGE PARK, Fla. — David Campbell switched on the overhead projector and wrote “Evolution” in the rectangle of light on the screen.

He scanned the faces of the sophomores in his Biology I class. Many of them, he knew from years of teaching high school in this Jacksonville suburb, had been raised to take the biblical creation story as fact. His gaze rested for a moment on Bryce Haas, a football player who attended the 6 a.m. prayer meetings of the Fellowship of Christian Athletes in the school gymnasium.

“If I do this wrong,” Mr. Campbell remembers thinking on that humid spring morning, “I’ll lose him.”

In February, the Florida Department of Education modified its standards to explicitly require, for the first time, the state’s public schools to teach evolution, calling it “the organizing principle of life science.” Spurred in part by legal rulings against school districts seeking to favor religious versions of natural history, over a dozen other states have also given more emphasis in recent years to what has long been the scientific consensus: that all of the diverse life forms on Earth descended from a common ancestor, through a process of mutation and natural selection, over billions of years.