So we have now got two potential methods. We supply and fit Clevertouch boards along with a VOIP system. The VOIP system has a paging facility so you call the extension of the paging number and anything you say in the handset is broadcast out of the loudspeaker/handsfree speaker on the handsets in the group, i.e. all the classrooms. If someone is on the phone, they don't receive it.
Clevertouch have a content delivery system called Cleverlive, where you can deliver content, i.e. alerts to all the boards that are on and in the school. They also have an MDM where you can WOL the boards from standby.
Food for thought, thanks for your input guys.