...Avoid buying any PLC's at at all. Trying to keep the students safe will eat up a lot of resources. Keeping all the equipment safe and working would eat up a lot of time. Eventually, they will let the magic smoke out of some of the gear.
I disagree, they need the 'REAL World' interaction to get their attention, remember these kids have played every kind of computer game that there is... they have seen everything imaginable on the screens, they need more to get them hooked.
As far as safety... all of my trainers are 24vdc, it's hard to get hurt and even harded to let the smoke out.
I do like the introductions to some sort of HMI/interface.