I remember this happening once in the first grade prior to our first fire drill; there were no announcements regarding such drills afterwards. I also remember two instances in the fifth grade where one announcement was for a fire drill (not our first one), and one instance where the fire alarm system was tested extremely briefly (half a second), likely following some sort of system work.
In the sixth grade, our first fire drill was announced over the intercom, and did not involve the use of any fire alarms; subsequent drills afterwards did, however. In the seventh and eight grades, fire drills (along with other emergency drills) were mentioned during morning announcements, explaining in which class periods they would occur (always close to the beginning, no more than 5 minutes). One drill I remember was announced to be happening in 30 seconds.
During high school, fire drills were normally not announced; most of the time, they would just remind teachers to discuss evacuation procedures to students. Teachers usually gave a heads-up regarding drills instead of the PA. There was one instance where the fire alarm system was being tested one day, and it was announced over the intercom system to disregard any activation during the system work. The alarms never did sound, however.
As somebody with Asperger’s Syndrome, I always became anxious for a fire drill, even when I knew one was coming.