That phrase you always hear when the good guys are faced with imminent danger that requires there participation in the defense of a particular strategic location. Of course, the big tough guy who is rudely waking the cadets from their slumber would probably say it even if it WAS a drill.
Manly Sergeant: Everybody out! This is not a drill!
Everybody Else: **Frantically puts on gear**
School Teacher: Everybody out! This is not a drill!
Kid: You know what? Somehow I doubt it. I think I'll just go finish up my chem homework while you guys go stand in the rain.