Last week, more than two dozen SWAT team members descended on Silver Lake Regional middle and high schools in Kingston, Mass.

Their weapons were drawn, but not loaded. They ran through the building, simulating their approach to a scenario in which a shooter or shooters were also inside.

It's called a "Code Yellow" and is meant to be a drill that prepares teachers and students for an unthinkable—but possible—reality.

Read the full CBS Boston story.