Lockdown drill
Lockdown drills are means of practicing preparedness in a business in the event of an intruder or criminal act. Generally an announcement is given that the building is going on an immediate lockdown. At that point, all occupants present at the time of the drill go to a room and lock all doors and windows tight. They must remain still until a "clear signal" has been issued. If the emergency is really life-threatening, then occupants present at the time of the emergency will evacuate to a meeting zone well away from the premises (scene of the emergency).
Frequency
Some schools, businesses, and agencies will conduct a lockdown drill yearly. Some schools may do a yearly lockdown drill as a replacement for or in addition to a fire drill.
Schools
Schools will do a lockdown drill one or two times per year. Some drills will be a standard lock-in until "clear signal" while occasionally an evacuation will take place. In an emergency depending on the severity, students may need to go home instead of returning to regularly scheduled classes. Some schools/counties have emergency codes. Sometimes a school may only be on lockout. Lockouts are when a criminal is close to the school. All doors that lead to the outside are locked; that way the criminal can't get inside the building. There are different levels. Level 1 represents an immediate threat; all students and teachers must hide and stop doing what they are doing. Level 2 represents a possible threat. All activities that require going outdoors are cancelled.