July and August are supposed to be the quietest months of the school year. But not this time.

In Williamson County, Tenn., protesters outside a packed, hourslong school board meeting last week shouted, “No more masks, no more masks.”

In Loudoun County, Va., a debate over transgender rights brought raucous crowds to school board meetings this summer, culminating last week with dueling parking lot rallies. The board approved a policy that allows transgender students to join sports teams that match their gender identity and requires teachers to use transgender students’ pronouns.

And, in a particular low point for school board-parental relations, a woman railed against critical race theory during a meeting in the Philadelphia area, yelling, “You have brought division to us.” After the allotted time, the school board president walked off the stage, into the audience, and took the microphone away. She was escorted from the lectern by security.

As summer fades into fall, nearly all of the major issues dividing the country have dropped like an anvil on U.S. schools.

“The water pressure is higher than it has ever been and there are more leaks than I have fingers,” said Kevin Boyles, a school board official in Brainerd, Minn., who said he recently received 80 emails in three days about face masks. He described being followed to his car and called “evil” after a board meeting where he supported a commitment to equity. Another time, a man speaking to the board about race quoted the Bible and said he would “dump hot coals on all your heads.”

“You are just trying to keep everything from collapsing,” Mr. Boyles said.

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This is hardly the first time the classroom has become the center of civil strife. From the teaching of evolution in the 1920s to the push for school desegregation in the 1950s, schools have often been a nexus for major societal conflicts.

“Schools are particularly fraught spaces because they represent a potential challenge to the family and the authority of parents,” said Natalia Mehlman Petrzela, an associate professor of history at the New School in New York City.

The two biggest divides in schools today are also highly volatile because they challenge fundamental narratives of what it means to be an American. The debate over mask mandates puts two values into conflict, collective responsibility versus personal liberty. And an examination of the country’s history of racism challenges cherished ideas about America’s founding.