Our school is currently using a system called Jatly. http://www.jatly.com
It is a smartphone app that can send Alerts for something that is urgent or messages for less urgent matters. These alerts and messages are sent to your local area or you can search for the hash-tag of your school to see all of the school messages. It is sort of like a local twitter.
Not only is it free, it can actually be used to earn IT equipment for your school.
There is more information on the site http://www.jatly.com
And also on the IT for Schools site http://www.itforschools.com.au/
If you would like any further info, just let me know.
This post has been edited by KAS: 27/02/2013, 01:15 PM