Newtown, Connecticut CNN  — 

High school seniors Emma Ehrens and Grace Fischer will hear their names read out today as they are called to collect their diplomas and start the next chapter of their lives.

But they are also steeling themselves to hear 20 other names of former classmates who never got to leave first grade, let alone plan for college and grow into adulthood.

Today is the day that the 20 children massacred at Sandy Hook Elementary School in December 2012 would have graduated, perhaps cheered on by the six teachers and administrators also killed in the deadliest K-12 shooting in US history.

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“It’s just going to be heartbreaking,” Grace told CNN. “I can’t imagine that 20 kids are not graduating with us and that they’re not having the opportunity to walk across the stage.”

Emma said she’s trying to prepare herself for a range of emotions – the joy at leaving school, nerves about what’s next and reflecting on what should have been.

“Thinking about all the what ifs: what if they were sitting next to me at graduation, what if we were still friends, where would we be? It’s just going to be a lot of what ifs in my head,” she said.

With gun violence the leading cause of death for children in the United States, and with shootings inside schools continuing to claim the lives of students and teachers while devastating communities in places like Uvalde, Texas; Parkland, Florida; Nashville, Tennessee; and Santa Fe, Texas – all since Newtown – many other students may be faced with their own “what ifs.”

Newtown High School’s Class of 2024 will be wearing green and white ribbons on their graduation gowns, each inscribed with “Forever In Our Hearts” to remember the children and adults who were killed while the new graduates survived, because they happened to be in a different classroom or got an extra few seconds to escape.

Emma Ehrens said her survivor's guilt has been intensifying in the run-up to graduation.

Emma was six years old and looking forward to making gingerbread houses when the 20-year-old gunman armed with a semi-automatic assault-style rifle and two handguns shot his way into the school. Emma, like many other students, thought the popping noises were from roof construction work at the school, until she came face to face with the horror.

“A guy – armed – came into my classroom and started shooting all of my friends and my teachers and my classmates,” she said. “His gun jammed and a friend of mine, Jesse Lewis, yelled at us to run and that’s what we did.”

Jesse Lewis, just six years old, shown in family photos with rosy cheeks and a smile full of baby teeth, was killed. But his quick thinking and desire to help his friends propelled Emma and a few other children to bump past the gunman and get out.

“I remember vividly running out of the room and then, in the hallway right by the main entrance, there were chairs … and under the chairs were people, because of the force of impact of the bullets, they were blown under the chairs.” She led her classmates out of the school and eventually was found by a family friend and taken to safety.

Grace Fischer says a large part of her childhood was taken away from her.

Grace hid with her friends in the cubbies in another classroom, feeling scared, she said, but not really sure about what, until police officers escorted them out to safety at the fire station next door.

“When we got to the firehouse, we had to line up by grade and that’s when my teacher was like, this is really not OK,” she said. “Because when we lined up by grade, half our grade was missing.”

That night, Grace learned that Jessica Rekos, a horse-loving fellow Girl Scout in her small troop, had died, though she said she could not comprehend what death meant then.

“That day really took a lot of my childhood away from me,” she said.

Both Grace and Emma clearly still carry the grief from the massacre but also seem propelled by their experiences to make their country safer.

Grace will attend Hamilton College in upstate New York, with plans to focus on law and justice. Rationally, she says it’s a choice she is making but she also acknowledges she feels an obligation.

“I really want to make sure that they know I’m doing something,” she said of her classmates who had no chance to graduate. “Even though they’re not here anymore, there are people who survived that are really trying to push for them. Because their lives were lost so early and I went through that at such an early age, I feel like it’s my purpose to continue my life in honor of them.”

Newtown High's Class of 2024 will wear green and white ribbons on their graduation gowns to remember those they lost.

Grace and Emma are part of the Junior Newtown Action Alliance (Junior NAA) to share their stories and campaign for gun violence prevention.

They’ve met with Vice President Kamala Harris and other national leaders, as well as survivors and victims’ families from other school shootings, perhaps driving Emma’s desire to become a “lawyer, senator, politician” after she graduates from her legal studies program at Roger Williams University in Rhode Island.

“I have thought about … if I’m not doing something big, did I survive for the right reasons?” she said, talking about the survivor’s guilt she feels she has had her whole life.

“When I was little, I don’t remember feeling it that much, but now that I’m graduating and going to college and doing all this stuff with Junior NAA, I’ve been feeling it a lot,” she said.

“I’m graduating, I’m going to college, I get to go home, I get to see my dog, I get to do all this when they don’t. It’s just really hard.”