My first elementary school: The alarms which were sounding in march time, would be silenced, then re-activated in continuous for three seconds to indicate the all clear. I don’t understand how that worked, but all I know is that it was a Simplex system,
Other elementary schools: I don’t know.
Last elementary school: All clear signal played over intercom system. Occasionally, they would ring the digital bell.
Middle school: I forgot.
High school: Announcement made over intercom.