Shoutbox

Well one easy method is by word of mouth, a headteacher tells every other teacher to tell pupils about some news. An assembly would be another way to do so.
However since this is in Tech Talk it could be IT project related, so I guess you'll want to know how to do it with tech.
Ok, you could print off letters for every pupil using a B&W laser printer with a fairly quick printing time. Another way would be a piped sound system, by either a WiFi or just a standard one, which uses speakers well placed to relay audio infomation from a single microphone.