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Twenty-four years after the firefighter Bill Spade still remembers every detail of being the last firefighter to escape the collapsing North Tower of the World Trade Center.

"I started getting buried and I said goodbye to my wife and I said goodbye to my two boys. And, almost 6-year-old at the time. And the other was two months old. When I had say goodbye to my two-month-old son, John, I said, 'You'll never know your Dad,' and that really hurt," Spade recalled of his harrowing escape.

Spade was also the sole survivor of Staten Island's elite Rescue 5 unit. 

In all, 343 of New York City's firefighters perished that day. When Spade's brother-in-law called him with the devastating news about his colleagues, the reality of what happened that day set in. 

"My brother-in-law Tommy had called me. And basically, he said, 'Bill, they're all gone.' And I named every guy that was having breakfast with that morning before the call came in. I named them once and I named them twice and he said, 'Bill they're all gone,'" he said.

 Now, with nearly a third of Americans born after that tragic day, Spade is racing against time to ensure future generations never forget.

According to current demographics, 122.4 million Americans were not yet born when the attacks happened or were too young to remember, prompting some to make sure that memories of Sept. 11 remain preserved.

Every summer, teachers from across the country gather at the  — located on the site where the World Trade Center towers once stood — to watch Spade turn his personal nightmare into a history lesson.

Among the educators from 30 states and abroad is Isabella Forget, a recent college graduate who will teach third grade in West Virginia.

"I was born in 2003, so I was, I don't have any memories of 9/11. I'm a teacher that's going into this kind of blindly," Forget said.

Teaching difficult subjects like terrorism to young children presents unique challenges, Forget said.

"They're just starting to become aware of what's going on in the world, and the world around them and how it works. So, to bring in that heavy subject matter is definitely difficult. But something that I'm dedicated to doing," said Forget.

For Forget, hearing Spade's firsthand account was transformative.

"It was one of the most meaningful things I think I've ever heard in my whole life," she said.

Spade believes teachers are crucial to preserving the memory of Sept. 11 because "they're gonna pass it on to the younger generation."

Throughout his children's education in New York's public school system, Spade would ask them each year what they learned about Sept. 11.

"What were you taught about 9/11 today?" he would ask. "And they all said, 'Oh, nothing, just a moment of silence.' And it would infuriate me every year that that's the most we can do is have a moment of silence."