The only ones I would trust with that task would be the Olympia SM8 and SM9. They're plentiful, reasonably cheap, very sturdy, easy running, have every feature you need, new enough to have good rubber, and they're usually found in very good condition.

Of course, there are other good typewriters, but you *did* ask for the *best (most durable)*.  There is no way i would expect any ultraportable to stand up to the job you describe. Get at least regular portables or consider desktop machines, of which many brands are good for this. Consider whether you want them all to be the same (for support and to minimize contention for "favorites") or to get a variety (to provide wider experiences and to allow demonstrations of different solutions to similar needs).

Have Fun! 

I don't know where the OP lives, and recommendations are most realistic when you know what's available locally. As for general recommendations, I agree with Michael that ultra-portables are definitely not a good idea. Larger, more contemporary portables would be the best choice; not only should they take the punishment, but they can still be easily stored when not in use.

I find that around here (northern CT) the 6-series SMCs are in abundance, and generally in good condition. Many more of them, it seems, than Royals. My caution with those is that their typebar linkages can pop out if subjected to rough use. Those are easily repairable, but maybe a sturdier mechanism, as on a standard, makes more sense. But as Uwe points out, storage space may dictate against standards.