Every school uses some sort of intercom system to communicate with the entire school. When used correctly, the intercom system can get a message out to hundreds (if not thousands) of students and staff at one time.
As an assistant principal and later as a principal I found "sweet spot" for using the intercom. My approach was pretty simple:
Find a time just before class lets out or right after class starts to make the announcement; announcements shouldn't last more than 60 seconds
Mix up the periods when you make your announcement so that you are alternating which teacher loses instructional time
Focus primarily on the most important activities (a big basketball game, a state speech contest, an upcoming dance); highlight individual student accomplishments when warranted
Be positive and use lots of energy and excitement when making the announcement
Pick and choose when you can embed and reinforce school rules, students expectations, or building goals into your announcement
By following these simple principles it turned the intercom into one of my biggest allies. Staff and students were actually saying "we like it when you do announcements" (when was the last time you heard that?!). I would get several emails and hand-written notes like the one I have copied below. Some might say a principal doesn't need to waste their time making announcements. I beg to differ - students love your involvement and it helps you keep a pulse on what is going on within your building.