CASTLE ROCK, Colo.—For the better part of four hours Tuesday night, residents of this Denver suburb stood at a lectern 20 feet from the seven members of the county school board, pointed their fingers and called them tyrants, sociopaths, liars and incompetents.

One armed sheriff’s deputy SAT in a chair a few feet from the podium. Two more hovered near the back of the room, and several other armed security officers stood in the hallway.

The...

CASTLE ROCK, Colo.—For the better part of four hours Tuesday night, residents of this Denver suburb stood at a lectern 20 feet from the seven members of the county school board, pointed their fingers and called them tyrants, sociopaths, liars and incompetents.

One armed sheriff’s deputy SAT in a chair a few feet from the podium. Two more hovered near the back of the room, and several other armed security officers stood in the hallway.

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The last school-board meeting before Election Day capped what has become one of the most vitriol-filled political contests in Douglas County in years, and is one of hundreds of school-board elections across the country that have turned into hard-fought political battles. Many of the elections set for Tuesday have become proxies for the larger culture war over masking mandates and the teaching of tenets of critical race theory.

In Loudoun County, Va., a school-board member resigned following threats of violence to her and her family. Florida districts are considering shortening public-comment periods at meetings, and in Kentucky a school board has asked parents to communicate through emails after board meetings became rowdy. Little of the contention is tied to the local issues such as building maintenance and teacher pay that usually animate school-board elections.

Across the 23 states that allow recalls for school-board members, 84 campaigns are targeting 215 board members, about four times greater than the 15-year average, according to Ballotpedia, a nonprofit, nonpartisan research organization based in Wisconsin.

A Douglas County sheriff’s officer monitored a school-board meeting. Board members sometimes have been the target of insulting language.

The number of candidates running per school-board seat increased by 17% from 2018 to 2021 across the 463 school districts monitored by Ballotpedia.

Attorney General Merrick Garland ordered the Federal Bureau of Investigation to work with federal prosecutors and local officials to thwart what he called “a disturbing spike in harassment, intimidation, and threats of violence” against teachers and school staff over politically charged issues, according to a memo Mr. Garland sent this month. Republicans have criticized the directive as an effort to silence parents who speak out on topics such as mask mandates and how race is addressed in schools.

In Colorado, 530 candidates are running for seats across 178 school districts, said Pam Benigno, Education Policy Center director for the Independence Institute in Denver, a libertarian-leaning think tank. In 2017, there were just over 200 candidates.

“I have never seen anything like this before,” Ms. Benigno said.

Douglas County, a stretch of small cities and towns south of Denver, is among the fastest-growing in the nation. The affluent, mostly white population has traditionally voted Republican. Newcomers are shifting the political balance. Republicans still outnumber Democrats by about two to one, but unaffiliated voters now make up the largest segment of the county. The district has 65,000 students. The student body is 82% white, 9% Latino, 5% Asian, 1% Black, and 2% two or more races, according to National Center of Educational Statistics.

The race for four seats on the seven-member board has boiled down to two slates: One favors masking mandates and training about social equity; the other opposes masking mandates as well as social-equity lessons which incorporate the underlying principles of critical race theory.

Legend High School is part of the Douglas County School District, where a spirited school-board race has focused on divided opinion over mask mandates and critical race theory.

Critical race theory, an academic concept first developed by legal scholars in the early 1970s, argues the legacy of white supremacy remains embedded in modern-day society through laws and institutions that shaped American society.

In March, in response to public calls for greater awareness of racism and sexism in the district, the board passed an equity policy which calls for tolerance, inclusion and a focus on social justice in schools.

Kevin DiPasquale, president of the Douglas County Federation, which represents teachers and other staff, said the school board’s focus on equity has helped to give teachers freedom to “teach fact-based history so they can prepare their students for the world ahead of them.”

Many parents spoke out to object at board meetings to condemn what they said was an overemphasis on race.

Mike Peterson worked from home last year during the pandemic as his daughter took her high-school classes remotely from home. Listening in, he said he thought the classes were light on rigor and filled with too much information that he considered liberal political opinion.

He started looking online to see what he could find out about the district, which led him to a video of a recent teacher training session in which he said diversity consultants explained that the U.S. was built by and for white, Christian, able-bodied, heterosexual men and that system was inherently racist.

“It was the teacher’s responsibility to bring it down brick by brick and law by law,” he said of the training. “I looked at that stuff and said, ‘Whoa, that’s it, we have to put a stop to that.’”

The district said it cut ties with the diversity consultants after many residents voiced objections.

Mr. Peterson, a 52-year-old retired Naval aviator, decided to run for school board. He also helped organize a slate of three other candidates he hopes will win a majority of board seats in Tuesday’s election. If the slate wins and takes control of the board, it has promised to get rid of mask mandates for students and pare back what Mr. Peterson believes to be political mission creep inside the schools.

Will Johnson, a Douglas County parent of three, who started a local chapter of the Foundation Against Intolerance and Racism (FAIR) supports Mr. Peterson. FAIR says it seeks to protect civil rights and contest the adoption of agendas that members believe focus too much on race and demands for social activism.

FAIR this year solicited complaints from teachers, students and parents of incidents from the local schools. They include: a seventh-grade teacher who played a podcast which suggested racism permeates the nation’s police departments; another seventh-grade teacher who wore a T-shirt that said “Stop Pretending Your Racism is Patriotism”; and a note from the school, titled “Condemning Racism and Social Injustice,” which included a call to “dismantle the colorblind framework.”

Mr. Peterson’s slate hopes to defeat, among others, board member Krista Holtzmann, the board’s vice president who is running for re-election with the support of many teachers. Ms. Holtzmann, a formal child-abuse prosecutor, has watched meetings grow more intense and angry over the past year-and-a-half.

Mike Peterson is part of school-board election slate that has signaled it would get rid of mask mandates if it wins seats in Tuesday’s election.

To de-escalate the tension, the board banned clapping. Instead, some parents stood at meetings and turned their backs. At one meeting, a man pantomimed punching another man with whom he disagreed. Police escort board members to their cars after meetings as a safety precaution.

“It just feels like the room reaches an intensity level where it does not feel safe,” Ms. Holtzmann said. “There is a loud vocal minority that have at times been inappropriate at best and threatening at worst.”

At Tuesday’s meeting, which began at 6 p.m., more than 70 people signed up to speak during the public-comment period. About a third congratulated the seven-member board for its patience and compassion. The rest were confrontational and angry.

“You have been weighed, you have been measured and you have been found wanting,” said Jason Kassay, a Douglas County parent.

“I hope it is the last time I have to see you,” Lauren Bostrom, another parent, told the board.

Just past 10 p.m., the board called a break, and a man wearing a shirt that read “A Masked Child is an Abused Child” stood near the back and loudly opined that forcing students to wear masks was unconstitutional. A woman, knitting in the front row most of the evening, raised her head.

“Shut up! Just shut up!” she screamed as she walked briskly toward the man demanding he leave the meeting.

A crowd quickly surrounded the woman and police cleared the room. A few minutes later, a few residents returned, and the board continued the meeting.

By 10:30 p.m. Ms. Holtzmann’s eyes were bloodshot.

She said she empathized with parents who care deeply about their children’s education but thought it was the board’s responsibility to keep students and staff safe and to provide in-person learning. She thinks mandating masks is the best way to accomplish that.

Promoting a diversity-and-inclusion agenda is important, she said, because pockets of bigotry and racism exist in the schools.

“I have heard candidates say they feel teachers are indoctrinating students and I find that offensive,” she said. “Teachers are professionals who choose a career not for the money but to help students. I don’t think they wake up in the morning set on indoctrinating anyone.”

Douglas County School Board candidate Mike Peterson and supporters waved to passing drivers on Wednesday.

She said she was frustrated by the level of animosity in the community. On some weeks she receives as many as 500 emails, many of them she considers toxic, she said.

Mr. Peterson watched the school-board meeting from the back of the room.

Mr. Peterson, a former assistant professor at the Air Force Academy in Colorado Springs, and the slate of three other conservatives with whom he is unofficially running for school board, have raised more $300,000, mostly from a few large local donations. The amount is among the most of any school- board candidates in the state and several times more than their challengers have raised, according to public records. The Douglas County Republican Party endorsed the slate as has 1776, a conservative group that supported former President Donald Trump.

Mr. Peterson said if he wins, his first step will be to go on a listening tour across the district schools, but he is adamant about redirecting the focus of education.

“There is an opportunity cost to teaching activism over academics,” he said. Citing test scores released by the state, he added, “Half the students in the district are not reading at grade level.”

Write to Douglas Belkin at doug.belkin@wsj.com