"You see troubled young men who are desperate and they strike out and they don't see that they have any hope," Bond said.
Schools generally are much safer than they were five, 10 or 15 years ago, Stephens said. While a single death is one too many, Stephens noted that perspective is important. In Chicago there were 500 homicides in 2012, about the same number in the nation's 132,000-plus K-12 schools over two decades.
"I believe schools are much safer than they used to be but clearly they still have a good ways to go," Stephens said.
The recent budget deal in Congress provides $140 million to support safe school environments, and is a $29 million increase, according to the office of Sen. Tom Harkin, D-Iowa, chairman of the Senate Health, Education, Labor and Pensions Committee.
About 90 percent of districts have tightened security since the Newtown shootings, estimates Randi Weingarten, the president of the American Federation of Teachers.
Many schools now have elaborate school safety plans and more metal detectors, surveillance cameras and fences. They've taken other steps, too, such as requiring ID badges and dress codes. Similar to fire drills, some schools practice locking down classrooms, among their responses to potential violence.
The incident involving the 5-year-old in Memphis led to the use of hand-held metal detecting wands inside elementary schools in Shelby County's school district.
Attention also has focused on hiring school resource officers, sworn law enforcement officers who are trained to work in a school environment, said Mo Canady, executive director of the National Association of School Resource Officers. He said his organization estimates there are about 10,000 of them in the U.S.
Canady said it was such an officer who helped avert more bloodshed at Arapahoe High School in the Denver suburb of Centennial when an 18-year-old student took a shotgun into the building on Dec. 13 and fatally shot another student.