OK, so next question - if you "image" by syncing all devices to the same machine initially, and then distribute to teachers, then teachers can't sync their ipads to their own laptops without losing the image, right?
You can pull apps from multiple iTunes store accounts to one iOS device. From the device log out of the 'default' iTunes store account from the settings app, then log in to another account. Then with that account get any apps that you want. Apps that have already been purchased with that account will download for free. I think you can have apps from 5 different iTunes store accounts on one iOS device.