What many consider America's first mass school shooting took place in the community of Littleton, Colorado, 20 years ago today. Two Columbine High School seniors killed 12 fellow students and a teacher, possibly setting the stage for the school shootings that haunt us to this very day.

For Kiki Leyba, who was in his first year of teaching and survived that day, there is no way to forget.

"I can see the silhouette of a gunman holding a long rifle. But I can also hear gunfire," Leyba told CBS News' Barry Petersen.

Leyba is still teaching at Columbine today, and when you talk to him you wonder: was it 20 years ago or just yesterday?

"Trauma has a memory," Leyba said, adding, "we can feel it."

Students still walk those same halls learning English and math and -- these days -- how to survive an active shooter.

"What was left behind in all of those years after Columbine and we just moved on?" Petersen asked.

"Trying to heal, whatever that looks like," Leyba said. "There's always the push to move forward, which drives me crazy. The largest group is that non-physically wounded, traumatized population. And then also the families that get lost in that as well."

After the 2012 murders at Sandy Hook Elementary School, their staff asked Kiki to come and share his insights and his memories.

"To see it on their faces, in their eyes, was so difficult," Leyba said.

"But when you spoke with those people and they saw that you were on the – I wouldn't say on the other side – but on the mending part of this, did that help?" Petersen said.

"Yes, I think it's a really important part of that healing, to have that opportunity to meet with somebody or talk to somebody who can relate to your experience," Leyba said.

Steve Siegel agrees. He was a victims' advocate for the Denver district attorney. "They're going through a time where their wounds have been ripped open," Siegel said. "It's flooding back."

Leyba's memories include a newspaper from the day after, with a headline that reads, simply, "heartbreak." He said at the time he had no idea how big or life-changing it would be.

He believes we need to continue the dialogue about what happened that day – even if it's painful.

"The memories. I mean, that's part of it. I mean, it's a packaged deal when it comes down to, you know, these mass shootings, school shootings. You're inevitably, you're gonna be dealing with that. And it's part of what we have to deal with until something changes."

Remembrance services in Littleton on Saturday are expected to draw a huge crowd of students, friends and survivors. It will be a tribute to the victims from Columbine. Many – like Leyba – may wonder: How many more of these before something changes?