Calls growing for more armed guards at schools following Texas shooting. But are they effective?

Many questions remain about how the gunman was able to enter the Texas elementary school.
According to officials, an armed school resource officer was the first to encounter Salvador Ramos at Robb Elementary School, but no shots were fired, yet, there are growing calls for more armed guards at schools.
An estimated 20,000 armed school resource officers are on duty, paid for by "close to $1 billion" from state and local governments hoping to stop violent attacks.    
But are they effective?
"Listen, I'm a parent of high school kids," says Marc Schindler, executive director of Justice Policy Institute. "If there was evidence to show that school police officers would make their schools safer, I would be all for it! But at the end of the day, there is literally no evidence to show that police in schools make schools safer."
Defenders of armed officers in schools say they must be seen as just part of a much larger, coordinated effort to make a difference.