Ah thanks for that, I finally understand! Single users can sync to as many devices as they like, establishments with multiple users must buy individual licenses. They're completely relying on the honesty of schools though, expecting them to buy multiple codes and then file them away basically unused while you sync your multiple devices to one account. Are they going to do audits of schools to ensure that they have their licenses?
The more I get my head around ipads, the more I realise that the software is quite unsuited to an educational setting. Apple are obviously keen to push their products to education, I think they really need to think about an educational version of iOS properly suited to schools, with decent teacher controls and more suitable syncing options. Mind you, with the way that schools are snapping up their products, why should it bother them?!