How can I send messages to all ipads

TBH, I'm not sure I would rely on something through the iPad to notify people of an emergency. Unless the emergency is something like "There is no power in the gym."

Check out Self Service and the associated videos: http://www.jamfsoftware.com/products/casper-suite/self-service/

iPads need iOS 8 and Self Service Mobile App (configured to your JSS)
then you can send a notification to the device>Self Service Mobile.
Just starting to test this, no feedback yet.
So far with middle schoolers the biggest challenge is getting them to accept the Self Service Mobile App install :)

We've been doing some work with this and it has gone pretty well. Make sure you set up the proxy certificate so that the notifications can also go to the lock screen on the iPad. You set this up in the same area as the push notification certificate in the JSS. As the same time we also enabled location services for the iPads by turning on iBeacons. I'd do all of this at once so that when a student opens the Self Service app they get both prompts and you can tell them to accept them. Of course, you can't get everyone but you'll get 'enough'. We timed this with rolling an app out via self service so that they'd get all of the prompts as a group which helps with getting them to answer in a useful way.

It's a pretty slick feature! The next thing we're looking to do with it will be things like teachers being able to send messages to their students in the evening like "Remember, chapters 2 & 3 need to be read before class tomorrow."