best method I have found is open an itunes account with a voucher card use an email such as email@example.com
buy an app or 2 then log in to each ipad and go to app store purchased apps page and download the app on each ipad
teachers can put there own apps on if they like using there own accounts.
Yes, you can download the app onto as many devices as you want. The app store on the ipad lets you sign in and out with what ever account you want and then download apps that were purchased with that account for free.
You can also use that same itunes library on a computer to sync everything over to as many devices as you want.
now on the ipods/iphones/ipads the limitation that exists is the number of itunes accounts.
Lets say your family has 4 itunes accounts. One for each parent and two children also have their own accounts. You would be able to download apps for free that have been purchesed on any of these accounts. However each device can only have apps and music from a limit of 5 accounts. So if your family has 5 accounts your still good and can load everything for free. If for some reason there are more than 5 account you will have to pick the 5 that you want to be able to use and stuff from the 6th(and up) account cant be used.
This is really great for those expensive apps like GPS turn my turn direction apps. Navigon North America is $79, I purchased it all the time to route around traffic. I'm also able to download it onto my family members devices for free just by signing into my itunes account and downloading it from the app store on the device.