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FORT COLLINS, Colo. (KDVR) — As the country gets ready to mark 20 years since Sept. 11, 2001, students in Fort Collins are getting a history lesson from a teacher who experienced the tragedy.

“What we’re going to talk about is emotional. I will probably cry,” Sarah Keller warned her students at the beginning of class Friday morning.

Keller is a U.S. history teacher at Fort Collins High School. A career change led her to the classroom 10 years ago, where she has included 9/11 as part of her curriculum ever since.

On Sept. 11, 2001, Keller was working for American Airlines in Texas.

“I knew the people who were the crew on the planes,” she said.

She told her students about the moment she saw a coworker lose his brother, who was working on the top floor of the south tower. The brothers were on the phone with each other when the building collapsed.

“I’m a person. I’m going to bring some bias. I’m going to bring some emotion and some subjectivity into this,” Keller said. “Because I did experience it.”

‘Our world dramatically changed on 9/11 and you guys are living that legacy’

Each of Keller’s students in the classroom Friday morning was born after the events of 9/11.

“You’re one of the first generations who was born after 9/11 and yet your world is deeply and incredibly shaped by 9/11,” she said.

Keller’s lesson focuses on the history of 9/11 and the way it has changed the world since.

“You guys live in a surveillance state that did not exist before 9/11,” she told the students.

According to Keller, students today are not as deeply connected with 9/11 as students in previous years. Still, she said that even after 20 years, it is one of the most impactful lessons she teaches.

“I can tell you as a teacher you very rarely get every single one of your students, especially in high school, paying attention. And every time I’ve taught this lesson they do,” Keller said.

Some students even got teary-eyed watching a video of witnesses reflecting on their experiences. They were free to write reflections during and following the lesson. According to Keller, many students wrote about the fear Americans experienced in the wake of the tragedy.

“They wrote about how they didn’t understand that fear, and that’s what really came through,” she said.

Keller said as the years go on, she will need to adapt her lesson for each class as students become more removed from 9/11.

“Our world changed. Our world dramatically changed on 9/11 and you guys are living that legacy,” she told this year’s class.