(CBS News) LITTLETON Colo. - Frank DeAngelis showed Columbine High School how to heal at a time when it was in unspeakable pain.
Now he prepares to step down as the school's principal, with too many memories he would rather not have.
When Frank DeAngelis retires next spring, he will be ending a 35-year career starting here as a teacher.
But Columbine is no ordinary high school; it was seared into a nation's memory on April 20, 1999.
Two Columbine students opened fire on their classmates, killing 12 students and a teacher. It haunts him still.
"The parents entrusted their children to me," DeAngelis said. "I felt I let 'em down."
English teacher Kiki Leyba went through the shooting and its aftermath.
"That day I watched Frank run towards gunfire," Leyba said.
Physically, Leyba said he was fine.
Emotionally, "I was coming apart," Leyba said. "And I went to him and I said, you know, 'Boss, I don't know if I can do this.' I can see the hurt on his face."
DeAngelis helped Leyba and stuck it out himself.
"They stayed with me and I stayed with them and we helped rebuild this community," DeAngelis said.
Now, with the principal who helped lead the healing is about to retire. "Pretty emotional when I think about it," Leyba said. "He's just been a rock."
DeAngelis' office is a frame shop of memories, from his early teaching days, to the memorials after the shooting that brought President Clinton here, to the wall for the lost.
A few feet away are the corridors where a dozen students lay dying. Those memories are finally fading.
"Now, all of a sudden, when I walk through the halls, I'm not envisioning them lying in a pool of blood," DeAngelis said, "but I am envisioning them playing volleyball, or high-fiving."
At the memorial near the school for the shooting victims, DeAngelies said he "most definitely" still sees their faces.
Columbine was the first mass school shooting. Now.
"Every time I get a phone call informing me that there had been a shooting, I'm thinking, 'Not again,'" DeAngelis said.
There are now many other parents and teachers and communities who need healing.
"The thing that allowed me to get through this, I could not allow hatred to build up inside of me," DeAngelis said. "Because if I did, it would just consume me, and I wouldn't be here today."
DeAngelis has one promise more -- that even in retirement, the victims will be in his thoughts very day.