The native of New York’s Lower East Side, whose older sister lived in Lower Manhattan, knew the area well. He also knew that small planes occasionally hit the towers.

“How little I knew,” said Smith, who soon learned of the severity of the situation and his thoughts turned to his sister’s fate. If she was at home or at work, she would be clear of the danger zone. But he couldn’t reach her.

It took more than a day to hear back from her and find out she was all right, he recalled.

“Even though I felt pretty confident that she was probably going to be OK, there was a part of me that was ‘Oh, my God. Oh, my God,’ ” Smith said. “And that feeling of total helplessness — I can’t go there, I can’t communicate with her, I can’t send help.”

Fifteen years later, those feelings help Smith to teach students about 9/11.

Smith now is chairman of St. Christopher’s upper school history department and one of the authors of the school’s “9/11 to Now” course.

As such, he has a front-row seat to the challenges of teaching a complex moment in our history whose repercussions still are being felt today and whose full impact may not be known for decades.

In Virginia, Sept. 11 and its aftermath are part of the Department of Education’s history and social science Standards of Learning curriculum. As part of their required studies on President George W. Bush’s term, students are taught about the attacks and the wars in Afghanistan and Iraq that followed.

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For teachers, the challenge of teaching about the Sept. 11 attacks is finding a way to engage a generation of students with little or no knowledge of the events.

“What changes for us as teachers is the things we take for granted: facts, understandings, legends, mythologies. The things we know, our kids know nothing about,” Smith said.

At St. Christopher’s, the school developed its course about eight years ago and requires that all seniors take it. The idea behind the course is to walk students through the story of Sept. 11 — what happened on the day through where are we now.

Smith said for teachers to engage students, they must start early in the semester working to make sure the kids connect with the subject matter.

One teacher focuses on United Airlines Flight 93. When passengers learned that their flight was part of a larger attack, they decided to fight back. As a result, the plane crashed into a field in Somerset County, Pa.

The idea, Smith said, is to hook the students by involving them in the personal stories of people on the plane and their families on the ground.

When he teaches the course, Smith uses amateur videos of how the attack affected New York City.

“Planes crashing into buildings, buildings collapsing, clouds of dust, people attempting to survive, tales of heroism,” he said. “All of those things that can help our students say, ‘OK, this is more than just an event in a history book.’ Which, for you and me, it’s not an event in a history book, but for them it really is.”

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Rob Williams, curriculum specialist for history and social science at Hanover County Public Schools, said county students learn the historic significance of Sept. 11 in seventh-grade U.S. history and 11th-grade U.S. history, as well as in world history, usually taken in the 10th grade.

Williams said individual teachers are able to develop their own lesson plans that address the historic significance “of a wide variety of events that are age-appropriate.”

“There is not a one-size-fits-all approach,” he said.

For Ashley Kelly at Tuckahoe Middle School in Henrico County, turning to the Freedom Flag Foundation was a way to develop a course aimed at her seventh-grade students.

Kelly, a social studies teacher, introduced the course in 2014 not long after the foundation, a local not-for-profit, approached the school about helping to teach students about the Sept. 11 attacks.

Teachers had realized they needed something that would help them teach their students about the attack. They had reached a point where their students either weren’t alive when it happened or so young that they had no memory of it.

The school, Kelly said, approaches Sept. 11 by looking at the heroes of the day and getting students to talk about what their idea of a hero is.

To help bring the events of that day to life, the foundation provided Tuckahoe with an “education trunk” that includes artifacts from the day of the attacks, including a piece of steel from the twin towers.

For their final project, students create a memorial video that includes an interview with someone who lived through Sept. 11.

Focusing on the heroes allows teachers to spotlight the resiliency of those who made sacrifices rather than the destruction wrought by the terrorists, Kelly said. And, more importantly, it allows the teachers to approach a subject that is very complex for middle school students in a way they can relate.

“They understand the sheer number of people who lost their lives in the destruction … and all the ways you can be a hero and all the different ways people of 9/11 were heroic,” she said.