The phrase Black Lives Matter first received national attention in summer 2014 and, since then, has become part of conversations on race in America. Here's how the phrase became a movement. (Claritza Jimenez,Julio Negron/The Washington Post)

It was picture day, and so like most high school students, Mariah Havard wanted to pick an outfit that said something — something about her summer, or who she would be in the year to come. Something that would make her face, in a sea of yearbook faces, stand out.

She pulled on green pants and combed her hair back into a bun. Over her head she tugged down a black T-shirt, marked with white lines between big, bold writing.

“BLACK LIVES MATTER,” it said.

Then the 15-year-old walked out the door to catch her bus.

But when she got to Buckeye Union High School, located in Phoenix’s westernmost suburb, Havard’s vice principal wasn’t as keen on her fashion statement.

A security guard escorted Havard to the administration office, she said, where she was told her black shirt and the message on it was disruptive to the school’s educational environment. The vice principal cited an incident from just days before, on Aug. 19, where the teen got into an argument with a white student who said, according to Havard, “that shirt is meaningless” and “black lives don’t matter.”

Next the teen was handed a plain white T-shirt, and she was told to change.

“She was asked to change and she didn’t question them,” Roxanne Havard, Mariah’s mother, told NBC affiliate 12 News. “She was being respectful.”

The teen walked to the bathroom to change, but first decided to snap a selfie, her Black Lives Matter T-shirt still on her body, the white replacement shirt wedged between her knees. She posted it to Facebook later that day, describing what had happened.

Havard called the white T-shirt “meaningless and non political” and said it “has nothing to do for what I’m standing for.”

“While wearing the shirt I have been verbally attacked MANY times,” she wrote. “I never meant to imply because black lives matter others don’t!”

What troubled her further was that she had seen classmates wear other political shirts that certain factions of society find offensive: supporting same-sex marriage, the confederate flag, white power.

To her knowledge, those students hadn’t been censored.

“At this point i’m starting to believe [there’s] a problem with the word black?” Havard wrote on Facebook.

The next day at school, Wednesday, the teen’s friend, Genesis Santoyo, wore a Black Lives Matter shirt to school at Buckeye Union. She, too, was told to take it off.

“I felt like I was being punished for who I am,” Santoyo, who is also black, told 12 News.

On Thursday, Havard’s mother met with administrators at the school, 12 News reported, and on Friday, Havard’s clothing was questioned once again.

A male administrator asked the 15-year-old to remove her sweatshirt, reported the Arizona Republic, in order to investigate whether she had worn the Black Lives Matter shirt again.

At one point, there was even talk that fights could break out over the controversy. Some students said they would wear Black Lives Matter shirts; others said they would come to school bearing confederate flags.

That day at school, Havard told Fox 10 that administrators announced neither shirt would be allowed in class.

As the incidents escalated, local civil rights groups began to take notice, and the teens started plotting.

On Monday, nearly a week after Havard was first asked to change her black shirt for a white one, she and Santoyo joined eight other students in a walkout from their morning classes.

“Stand up,” they chanted while exiting the building, “fight back.”

They wore Black Lives Matter T-shirts and carried white signs with red lettering that said “vision for black lives.” In some photos, the teens raised their fists in the air, a symbol of solidarity.

“We’re not trying to start a race war,” Santoyo told the Republic. “We’re trying to end one.”

The students were joined by their parents, and local politicians and civil rights groups voiced support for their cause.

Havard’s mother, Roxanne, embraced her daughter before addressing the small crowd outside the school Monday, according to the Republic.

“When they wear their shirts that say Black Lives Matter, they’re just telling you that their lives matter, too,” Roxanne Havard said.

“They have made death threats to the students here on Snapchat, Facebook, social media,” Roxanne Havard said. “The students that have made these threats have not had any consequences.”

Representatives from Black Lives Matter Phoenix, the NAACP and the ACLU encouraged the students to voice their grievances in an intelligent, nonviolent way, the Republic reported, and Iisha Graves, a state congressional candidate, said she felt proud they were standing up for what they believed in.

Just after noon, the student returned to class.

The school administration would not answer specific questions about their dress code policies, according to local media, but did release a statement on its website explaining the dress code policy:

“The Buckeye Union High School District is committed to student learning and campus safety, and will continue to enforce school policy to ensure a safe and successful learning environment. In regards to the specific situation, the district strives to remain politically neutral while still allowing student expression. However, when these expressions interfere with the learning process and become a potential danger to students, they have to be addressed. We appreciate the community’s support and will make every effort to offer the best educational programs and opportunities for all students.”

The Buckeye Union school district further addressed the situation in another statement on its website and Facebook, this time focusing specifically on the protest and its aftermath. The district said it was working with its staff, local leaders and Black Lives Matter representatives to “turn the incident involving the … T-shirt from a negative situation into a positive learning experience.