The schools in my area don’t conduct fire drills during lunch periods.
There was one time at an elementary school I worked at when the fire alarm sounded during lunch and they made a PA announcement saying “everyone stay put while we investigate the source of the alarm.” It was a faulty duct detector in the fifth grade wing from what I heard on the walkie-talkies. They then announced an “all clear” announcement on the PA system. A few minutes the later the fire alarm sounded again for a few seconds. They made an announcement to “disregard the fire alarm”. A few minutes later the fire alarm sounded for a third time and they made an announcement saying that they were going to evacuate the building. Apparently after the fire alarm sounded for the third time, they had to evacuate the school and have the fire department come investigate. No issue was found by the fire department. While we were outside the fire alarm sounded again for a fourth time for a few seconds.
They rescheduled lunches due to the evacuation.