We currently have a tannoy system that goes through the entire site. 200-250 speakers (Bosch Praesideo), a few years back we installed a lockdown based system which uses multiple CQR PADP3/SS/BK Double Press Panic Button's across our site, these feed a relay which will broadcast a message over our tannoy system. Then when the key releases the panic buttons, an all clear message is broadcast.
We use this on a daily basis to advise students of important messages, request they go to a location, such as the principal's office (very american, i know) and reminders of key events. Well worth the investment.
------ And you can even plug in an iPod to play music site-wide, or in specific zones!
There are a few things that I've seen over the years... and perhaps some suggestions...
- Either investing in a tannoy system as above
- Do you have phones in every classroom? Can you create a paging group? We use 3cx and it's easy to setup! - this however will miss common areas such as corridors
- Digital Signage, and morden boards can display a message - although would take some time to action and sync, and not the most time-critical solution.
- Upgrade your fire alarm system to a system that can have multiple sounds or sirens - this may make the most financial sense to get everywhere, as you have the cabling there.