Greg Hill, the head football coach at San Francisco’s Mission High School, was standing on the sideline Saturday afternoon before a game in Marin County when it occurred to him that a player or two might take action during the national anthem.

As the song began, Hill, 41, turned toward his team and was taken aback by what he saw.

Every player on the roster — black, white, Latino, Asian — was on one knee, an echo of the hotly debated move by 49ers quarterback Colin Kaepernick to protest racial inequality and police brutality. In that moment, Hill said, he saw a team that had decided to experience “this historical moment” as a unit.

The coach, who is black, also had a more immediate dilemma: What am I going to do?

“I decided I’d stand for them,” he said. “I’m gonna stand for my team.”

The players at Mission High plan to kneel again at their game Friday night at San Mateo High, and they won’t be alone. In the weeks since Kaepernick sat, then knelt, for the national anthem, scores of professional and amateur football, soccer and volleyball players in fields and courts across the country have joined in.

The protests have been criticized by people who see them as an affront to the country and its military veterans, and defended by others who either applaud Kaepernick’s message or simply support his right to deliver it.

But the larger trend is that of athletes awakening to the power of their stage, said Jeremi Duru, a professor of sports law at American University’s law school in Washington.

“Throughout the nation, athletes on different levels are finding their voice and recognizing that they have a platform,” he said. “We haven’t seen this level of athlete activism in nearly half a century. This is a movement.”

Last week, more than a dozen NFL players, including the 49ers’ Kaepernick and Eric Reid, knelt or raised their fists during the anthem. High school football players in Kentucky, Massachusetts, Maryland and other states also knelt.

The athletes’ reasons for protesting have varied, as have the responses. While the 49ers pledged $1 million to aid causes cited by Kaepernick, Denver Broncos linebacker Brandon Marshall lost two endorsement deals after he knelt during the anthem.

A high school football player in Massachusetts who took a knee said he was hit with a suspension, though school officials soon announced that he would not be disciplined. A private Catholic school in New Jersey said it would suspend players who did not stand for the anthem.

But at public schools like Mission High, students have constitutional protections for peaceful, nondisruptive speech and action such as sitting out the national anthem or the pledge of allegiance, said Frank LoMonte, executive director of the Student Press Law Center in Washington.

In California, a private school student who protested the anthem would be protected as well by the state education code, he said.

“A student in California has the legally protected right to engage in any speech that would be considered constitutionally protected if it took place in the off-campus world,” LoMonte said.

The silent protest by the Mission High football team wasn’t spontaneous, nor flippant. According to the players, the conversation over whether to do it began in earnest between a few players during Saturday’s bus ride to Redwood High in Larkspur.

Then, before the game, Niamey Harris, a 17-year-old senior team captain and the starting quarterback, along with a few teammates, explained to the rest of the players that they would take a knee to bring attention to racial injustice in America. They asked their teammates to join in so they could do it as a unit.

The pitch to the other players, Harris said, went like this: “This is for helping everybody else in the world to understand that black people and people of color are going though difficulties and they need help. It’s not going to take care of itself.”

Harris, who is black, is from Hunters Point, a San Francisco neighborhood long scarred by poverty. He lives a few blocks from where city police officers killed 26-year-old Mario Woods in December, a video-recorded shooting that sparked protests across the Bay Area and a U.S. Department of Justice review of the police force. Harris regularly took the bus that had stopped beside Woods before Woods was shot.

Watching the video hurt, said Harris. He grew up watching people get “messed with” by police, he said, and has been harassed himself for no reason.

“It’s just hard to go through that stuff,” he said. “You go through a lot of stress just thinking about it.”

Teammates Marvin Pusung-Zita, a 17-year-old junior who plays defense, and sophomore offensive lineman Brindan Shepard, 16, said they knelt to support their fellow players.

“I consider my teammates my brothers, and being that my brothers on the team are black and oppressed, then of course by all means I’m going to support them and take the knee,” said Pusung-Zita, whose family is from the Philippines. Shepard is white.

They and other players, including Kaepernick, sought to explain that their action was not anti-American.

“I’m the first born in the U.S. from my family,” said Pusung-Zita. “It’s an honor to live in America, but I feel like this is just something we have to fix.”

So far, the Mission High boys have received support — Pusung-Zita got a round of applause during one class — and plan to continue their protests throughout the season, at least at games that feature the national anthem.

Coach Hill said he was often moved to tears by the anthem during his own playing days because it represented the freedom of being an American. But he understands and respects his team’s decision. Taking a knee is a sign of respect, he explained, while also acting as a symbol of protest. He noted that after practices, his players get down on one knee to listen to his instructions.

“It’s important that the whole team did it,” he said. “We’re a diverse team — black, white, Hispanic, Asian — so the fact they all did it? I could only respect them for doing that. They’re the future leaders of this country ... so for them to lead the way, it shows change can take place, and it starts with them.”

Hamed Aleaziz is a San Francisco Chronicle staff writer. Email: haleaziz@sfchronicle.com Twitter: @haleaziz