Headteacher

We are a school who need to develop a "lockdown" protocol in the event of an intruder on site. We have two buildings, close together, each of two floors, so sending messages verbally and crossing between buildings takes an inordinate amount of time. In theory, with a powerful enough wireless transmitter, and probably some wireless extension relays, a simple alarm/doorbell system would do the trick. Wireless sirens are available at <£20, doorbell chimes even less. The technology seems to exist (ie multiple sensors feeding one alarm) but we want to reverse this. Commercial/wired solutions cost a silly amount of money. Can anyone suggest how to develop an extendable wireless system which would allow one or two buttons to make around 20 receivers/sirens/chimes activate?