A 20-year veteran teacher is accused of assaulting a middle school student who refused to stand for the Pledge of Allegiance, the latest incident in which a protest over the flag has devolved into apparent violence.

The teacher is Karen Smith, who until recently taught physical education at Angevine Middle School, about 20 miles north of Denver. She couldn't immediately be reached for comment.

Officers were called to the school around noon on Thursday, The Denver Post reported. Neither the school nor the police released details, and the student's name has not been released. No charges have been filed.

A school spokesman told the newspaper that the policy is to allow students to sit or stand during the pledge.

In a letter to families obtained by Denver CBS affiliate KCNC-TV, Principal Mike Medina told families that “we will have a substitute teacher working with some of our PE classes for the time being.”

He added that Smith had been placed on paid administrative leave and that the school was “working closely with our partners at the Lafayette Police Department.”

Protests involving the American flag have become a flash point in the last two years since then-NFL player Colin Kaepernick knelt during the national anthem to protest racial injustice.

His example was followed by other NFL players, athletes in other sports and countless people with similar stances.

Those who kneel see the act as a powerful, nonviolent way to protest racial injustice, particularly society's inaction when police kill unarmed African-Americans.

Critics say it is an unacceptable assault on the values and mores of the United States - and its most potent symbol. Others in this camp say the protests are a disrespectful slap in the face to veterans of all races who have fought or died for American freedoms.

The differences of opinion frequently boil over into the public sphere, debates that are always contentious and sometimes violent.

In October, a woman was recorded throwing her drink on two men who knelt during the national anthem at a Los Angeles Lakers game, according to The Daily Mail. “Excuse me, this is for the national anthem you pieces of s***,” she yelled as she tossed the beverage. “Disrespect our flag and our country, and that's how we'll react.”

The Long Island Diocese of Rockville Centre (New York), which runs a Catholic school system, said in September that students who kneel during the anthem would face “serious disciplinary action,” according to The New York Times. Athletes who didn't “stand in a respectful manner” could see reduced playing time or be kicked off the team.

Angevine Middle School, Colorado (Google Maps)

In October, a Native American high school football player successfully sued a California school district, which required students and staff to stand and remove their hats when the anthem was played, according to Yahoo Sports.

“Students like our client who conscientiously carry their values and ideals with them cannot be silenced or directed on what to say or not say by their school in this manner,” Katie Traverso, an attorney for the student whose identity was not revealed in court documents, told the news organisation.

Last year, President Donald Trump injected himself into the kneeling controversy.

In a September speech in Alabama, President Trump called for NFL owners to suspend or fire any “son of a b****” who doesn't stand for the national anthem.

The remarks touched on an exposed nerve in a league where most of the players are black and most of the fans are white.

American sportspeople kneel in protest Show all 14 1 /14 American sportspeople kneel in protest American sportspeople kneel in protest Terrance Smith #48, Eric Fisher #72, Demetrius Harris #84, and Cameron Erving #75 of the Kansas City Chiefs is seen taking a knee before the game against the Los Angeles Chargers at the StubHub Center on 24 September, 2017 in Carson, California Sean M. Haffey/Getty American sportspeople kneel in protest Members of the Dallas Cowboys link arms and kneel during the National Anthem before the start of the NFL game against the Arizona Cardinals at the University of Phoenix Stadium on 25 September, 2017 in Glendale, Arizona Christian Petersen/Getty American sportspeople kneel in protest Bruce Maxwell of the Oakland Athletics kneels as teammate Mark Canha #20 places his hand on Maxwell's shoulder during the singing of the National Anthem before their MLB American League baseball game against the Seattle Mariners at Oakland-Alameda County Coliseum in Oakland, California on 25 September 2017 Reuters/Stephen Lam American sportspeople kneel in protest Members of the Oakland Unified School District Honor Band kneel as they perform the national anthem before a MLB American League baseball game between the Oakland Athletics and the Seattle Mariners at Oakland-Alameda County Coliseum in Oakland, California on 25 September, 2017 Reuters/Stephen Lam American sportspeople kneel in protest Samaje Perine #32 and Chris Carter #55 of the Washington Redskins hold hands as they stand and kneel in unison during the national anthem before playing against the Oakland Raiders at FedExField on 24 September, 2017 in Landover, Maryland Patrick Smith/Getty American sportspeople kneel in protest Meghan Linsey, a former contestant on "The Voice," kneels after singing the national anthem before the start of the Tennessee Titans and Seattle Seahawks football game at Nissan Stadium in Nashville, Tennessee on 24 September, 2017 Ryan Hermens/The Paducah Sun via AP American sportspeople kneel in protest The Los Angeles Chargers are seen during national anthem before the game against the Kansas City Chiefs at the StubHub Center on 24 September, 2017 in Carson, California Jeff Gross/Getty American sportspeople kneel in protest Nick Kyrgios of Team World joined NFL stars from Wembley by dropping his knee before his Laver Cup match with Roger Federer today during his mens singles match against Roger Federer of Team Europe on the final day of the Laver cup on 24 September, 2017 in Prague, Czech Republic Julian Finney/Getty American sportspeople kneel in protest Laremy Tunsil #67, Maurice Smith #27 and Julius Thomas #89 kneel with Jarvis Landry #14 of the Miami Dolphins during the National Anthem prior to an NFL game against the New York Jets at MetLife Stadium on 24 September, 2017 in East Rutherford, New Jersey Steven Ryan/Getty American sportspeople kneel in protest Tampa Bay Buccaneers Wide Receivers Mike Evans #13, and DeSean Jackson #11, take a knee during the national anthem before the game against the Minnesota Vikings on 24 September, 2017 at U.S. Bank Stadium in Minneapolis, Minnesota Hannah Foslien/Getty American sportspeople kneel in protest Denver Broncos players kneel during the American National Anthem before an NFL game against the Buffalo Bills on 24 September, 2017 at New Era Field in Orchard Park, New York Brett Carlsen/Getty American sportspeople kneel in protest Buffalo Bills players kneel during the American National anthem before an NFL game against the Denver Broncos on 24 September, 2017 at New Era Field in Orchard Park, New York Brett Carlsen/Getty American sportspeople kneel in protest Colin Kaepernick #7 and Eric Reid #35 of the San Francisco 49ers kneel in protest during the national anthem prior to playing the Los Angeles Rams in their NFL game at Levi's Stadium on September 12, 2016 in Santa Clara, California. The kneel began as a protest against police killing people of colour Thearon W. Henderson/Getty American sportspeople kneel in protest Megan Rapinoe #15 kneels during the National Anthem prior to the match between the United States and the Netherlands at Georgia Dome on 18 September, 2016 in Atlanta, Georgia Kevin C. Cox/Getty

As The Washington Post's Cindy Boren wrote in November, the NFL doesn't require its 1,600 players to stand. Owners, who met in October with players to discuss the issue, have declined to compel them to do so.

After Trump's statements, NFL players - and in some cases owners, coaches and staff - engaged in a collective show of solidarity, standing with arms locked or kneeling or remaining in the locker room during the anthem.