In comfortable suburbs such as this one on the outskirts of Denver, school board meetings are normally sleepy events.

But at last week’s session of the Jefferson County Board of Education, hundreds of people lined up two hours in advance to get in. One man waved a copy of George Orwell’s “1984” at the board. Two high school students hauled in cardboard boxes containing 40,000 signatures to a petition they had circulated online. Another one told the five-member panel, “America was founded on what you are trying to prevent!”

Jefferson County has become ground zero for a new culture fight — this time over how to teach U.S. history to high-achieving 10th-graders.

At issue are changes to the Advanced Placement history course, one of the academically rigorous classes high school students across the country take in hopes of earning college credit and impressing admissions officers at selective schools. Last year, nearly 440,000 students took the AP history exam, one of the most popular AP tests offered.

The College Board, which administers exams to students upon the completion of AP courses, has revised the history curriculum in ways that have angered conservatives, who say it paints a darker picture of the country’s heritage and undervalues concepts such as “American exceptionalism.” ...