Migrate back to Mountain Lion

Hi all,

I have a small issue at hand - as a school we lease our Mac's and they are all preloaded with licensed software, however new replacement machines are running Mountain Lion. I cannot upgrade them without an Apple ID (the particular image is not supposed to ask for an ID, but it does in error)

Now some of my clients have gone and upgraded to Mavericks on their leased machine. I don't seem to be able to use Migration Assistant (via network) to Migrate from Mavericks -> Mountain Lion. The machines do not find each other. Is this by design?

If so its unfortunate, because I am only interested in migrating documents and leaving applications and system settings behind. Migration doesn't give me a 'document only option' during OSX downgrade, it just doesn't find the other machine.

Time backup the only option in this case? (or trying to upgrade target machines to Mavericks?)