Couple of things, born of years of teaching.
A) Get some canvas, and make a cover for it. Keeps dust, crud, and bored fingers off of it.
In a perfect world, rig up some system whereby the cover can be locked on. Keeps it out of action when you're not around. Which prevents all sorts of 'seemed like a good idea at the time' projects from happening.
Out of sight=out of mind. Which is a good thing.
B) if you can't rig the cover to lock, get a bit of chain and a lock, and lock down (or remove) the handles.
C) Make sure the top center gear can't come out. If it can, make sure you know how to realign the rolls when some genius gets the rolls out of step with each other. Because they will.
D) find some open cell foam, like they use in couch cushions. Cut a strip that'll span the width of the rollers, and make one that's tall enough to tuck under the bottom roller, and another one that'll tuck on top of the upper one, with enough height remaining to make sure it stays in place with the upper roller all the way down in contact with the lower one.
Then soak the contact area in 3-in-1 oil, sewing machine oil, or some other similar light oil. The idea is that the foam sweeps crap off the rolls as they turn, and keeps the rolls oiled, both of which are good things. Pull the foam off the rolls if they'll be out of action for more than a week or two. Long term contact in one spot may be problematic, depending on the foam. If it's turning regularly, not a problem.
Find and buy a can of LPS-3. It's spray on rust inhibitor. Great stuff. Especially good for over-the-summer storage.
Comes off with WD-40, which shouldn't be used for any serious oiling. It's not nearly good enough for protecting the rolls in normal use, for example. (It's basically kerosene, and evaporates away to nothing almost immediately.)
You haven't mentioned what age group you're working with. That has a lot to do with what you tell them, and what you show them.
Hope this helped,