LITTLETON, Colo. – Change the sound of the fire alarms. Consider banning what the cafeteria cooked for lunch that day. Watch out for slamming lockers and popping balloons. And accept that normal will never truly be normal again.

That’s the advice survivors of the 1999 Columbine High School shooting have for their fellow students, educators and administrators in Florida’s Marjory Stoneman Douglas High School. Nearly 20 years after Eric Harris and Dylan Klebold killed 12 of their fellow students and teacher Dave Sanders, the lessons of Columbine are visible across the nation: more metal detectors and armed guards, a more aggressive police response.

Less visible but no less important are the lessons learned by the teachers and administrators who lived through that day, and returned to their shattered school weeks later to pick up the pieces.

At Stoneman Douglas, curriculum will be altered, grading diminished, teaching style adapted. Test scores will drop and learning will slow, at least in the short term, as the traumatized students and staff try to readjust to a campus transformed into national focal point on gun violence.

"Everybody needs school to feel like school. But everyone is traumatized and traumatized brains don't learn well," said Paula Reed, an English teacher who survived the Columbine attack. "It's going to be a while before they even start getting their feet under them."

Monday and Tuesday will be staff planning days, and classes resume Wednesday on a modified schedule. The full class schedule is set to resume March 5. The freshman building, where the carnage took place, will remain closed, and tentative plans call for its demolition. The school has been closed since Feb. 14, when authorities say former student Nikolas Cruz killed 14 students and three staff members.

Now retired, Columbine principal Frank DeAngelis has already spoken several times with Ty Thompson, the Stoneham Douglas principal, offering hard-earned guidance and advice. DeAngelis has become one of the world's leading experts on school violence and its effects on communities.

"It's the things you don't even think about, things that will trigger the emotions," DeAngelis said. "Teachers won't know what to expect. It's a day-by-day experience."

Frank DeAngelis, principal of Columbine High School in Colorado, looks at what he calls his Wall of '99 in his office at the school (Photo: 9NEWS/KUSA-TV).

DeAngelis said the smell of the food from the cafeteria could trigger memories for some students, and the fire alarm sound certainly will. So might sirens or even the sound of a slamming door. Even camouflage clothes or the sight of multiple police cars parked out front could frighten kids, he said.

After the Columbine shooting, well-meaning community members built a balloon arch in the school colors to welcome students back to class. “What we didn’t anticipate was balloons popping and kids diving on the ground,” DeAngelis said.

Stoneham Douglas teachers will bear some of the heaviest burdens, DeAngelis and Reed said, because they'll try to be strong for the students while still struggling themselves. DeAngelis said he always recommends counseling for teachers following a shooting, so they can help support their students.

"If they don't take care of themselves, it will be impossible for them to care for anyone else," DeAngelis said. "They’re going to want to put on a tough face for the students, but they are also hurting. You have to remember that you're not in this alone."

In the weeks after the shooting, Reed remembers walking into her home, past her husband and two kids, to sit on the bed and stare out the window, exhausted after each a day of trying to teach and balance the emotional needs of herself and her students.

Reed said at first, she altered her teaching style: Instead of lecturing or asking for class participation, she asked her 10th and 11th-graders to follow along as she read aloud to them. The comforting familiarity helped, she said, as did a daily check-in where she asked each student to rate out loud how they were feeling. For the first few years, she also avoided teaching books full of violence or teen deaths.

The most important thing, Reed and DeAngelis said, is to remember that every student and staff member will be at a different place in their grieving process. Some will be ready to move on quickly, the two said, while some won’t be able to stop talking about it, and others will refuse to discuss it at all – at least at first.

Many will struggle with guilt, knowing they survived while their classmates and colleagues didn’t.

“The first few good days you have can also be the hardest,” Reed said. "You think you see the light at the end of the tunnel and it's actually an oncoming train."