But in recent years, schools also came to value natural light and open spaces that allow classes to collaborate. Walls once made of brick are now made of glass. They fold up and disappear, if they're there at all. It's better for learning but tougher for security, Nigaglioni said.
"It's becoming a challenge," the Dallas-based architect said. "We're breaking down the school into more small learning communities, the school within a school. You might not have a wall, so securing your hallway or pod is how you do it."
If there's a shooting, an explosion, an illness, it can sometimes be limited to one room or one hallway. At Sandy Hook, students were killed in two classrooms.
But plenty of schools don't have even those options. John Kuhn is superintendent of the Perrin-Whitt Consolidated Independent School District, a rural district of about 350 kids northwest of Fort Worth. He spent the afternoon celebrating Christmas with his elementary school students, pretending not to know the news.
"Oh my gosh," he thought. "I have to take care of these kids."
His schools will review their crisis plans and drills next week, he said. But there's no school law enforcement, and because it's a rural area, any police response could take a while. His middle and high school students circulate between five buildings during the school day, so they can't lock doors.
Security systems cost money, he said, and when he's cutting teachers, how could he hire a police officer?
"Days like today make you think, is that the right decision? How can we make our systems airtight?" he said. "You know ultimately, you can't."
After Columbine, after Virginia Tech, after Sandy Hook, security experts said that will be the hard reality for every school, politician and parent to accept.
"There is not a single safety measure that anyone could have put in place at that school that would have stopped what happened," said Bill Bond, the school safety specialist for the National Association of Secondary School Principals. "When you allow absolutely insane people to arm themselves like they're going to war, they go to war."
He calls metal detectors useless. Buzzer systems are just locked doors. Lockdown plans are important to keep people safe, but they don't keep evil out.
"In a school, your only real protection is kids trusting you with information," Bond said. "If they don't trust you with information and someone is planning to do something, it's a matter of how many will be killed before you kill him."
When it comes to shootings, Bond calls himself a cynic. Over the course of 12 seconds in 1997, one of his students shot and killed three classmates, wounded five more, then put the gun in Bond's hand.
Bond was principal at Heath High School in West Paducah, Kentucky, then. The shooter was 14. Michael Carneal was later sentenced to life in prison.
Bond stayed on at Heath long enough to see survivors graduate. The school's culture changed overnight, he said, to one of support and trust. After three funerals, the bullies changed.
He retired and built a second career around talking to high schoolers about security. He tells them to pay attention to what they hear and tell people who can help. That, he promises, works.
He's not in Newtown yet, he said, because, well, what could he say to 6 year olds? What could he say to their parents to make this better?
Nothing, he says. Not a thing.